In Vintage, when a card is too good it gets restricted. It doesn't matter if that card is Black Lotus good or just Grim Monolith good, the result is the same. With the unbanning of Channel and Mind Twist in 2000, the DCI swept away the remnants of policy from years past and ushered in a new era in Vintage where restrictions would be the only way to keep things in check.
The principal behind the restriction policy is that by limiting a card from a four to a single copy in deck construction, the DCI can destroy overly dominant or distorting decks that rely on multiple copies of the card in question. The majority of the restricted list exemplifies this policy. In recent terms, think of GroAtog with four Gush or Long.dec with four Lion's Eye Diamond. Restriction works.
There are two reasons why restriction is an effective policy device. First, with only a single copy, the card won't come up with enough frequency in a sixty-card deck to rely on it or abuse it to the full extent. The restriction of Fact or Fiction killed BBS and the restriction of Gush ruined GroAtog. Second, restrictions make the card itself significantly weaker. There are hidden synergies that arise from being able to design a deck around a four-of. This is obvious with Mind's Desire but take cards like Channel, Black Vise, Gush, and Fastbond for example. All four cards are hugely powerful but see almost no play because they are narrow cards that don't have a good home as singletons. When you can't rely on a card, decks have to find other engines to rely on making powerful, restricted singletons like Fact or Fiction less important.
Yawgmoth's Will is the one card where the logic of restriction I have just presented breaks down.
(1) Restriction Has Not Sufficiently Neutered Yawgmoth's Will
In the first place, Yawgmoth's Will is not a card that you need multiple copies of, or need immediately. The resources spent leading up to Yawgmoth's Will are what make it so powerful. The strength of a Will is conditioned on the strength of the cards already in the graveyard and the strength of other cards in the deck. Therefore, a strong Will is preceded by other spells that increase the chances that Will will be found and resolved. In other words, unlike most restricted cards, Will is not a card that you can heavily diminish the influence of by restricting because decks aren't trying to get Wills in their opening hand so that it can be the first spell they cast. Going Swamp, Dark Ritual, Yawgmoth's Will are your first play is as funny as it is useless. However, going Swamp, Dark Ritual, Necropotence is nothing to laugh about.
This force of this point is brought home with the realization that each powerful preceding spell increases the likelihood that Will will show up - and when it does, it will grow more broken with each spell cast. It is for this reason that decks with robust drawing engines often don't even need to Tutor for Will - if you simply draw enough cards you will find it.
Yawgmoth's Will is called "Yawg Win" because that is generally what happens when it resolves. The effect is to speed up the format by permitting decks to "combo" out far earlier than they would normally win. It seems obvious to me that decks will become more and more efficient at abusing this card, until, once again, something else needs to be restricted. JP Meyer has compared Yawgmoth's Will in Type One Tog to Upheaval in Tog decks of other formats. The idea that the game ends there is right on - but what the analogy misses is that the spells that led up to the casting of Yawgmoth's Will have 1) made it more likely to resolve (because you have more countermagic to protect it from having drawn so many cards) and 2) made it more broken (by filling up your graveyard with juicy spells). Upheaval is more analogous to Berserk.
This isn't to say that people wouldn't play with four Wills if it was unrestricted - I have no doubt that they would, but the assumption underlying the principle of restriction that you want to diminish the number of multiples in a deck in order to stop a certain engine is less applicable to Will. The power of Yawgmoth's Will is not really affected all that much by restriction. Sure, quite a few decks would run four Yawgmoth's Wills if they could (Long.dec did so vicariously through Burning Wish), but many would run only two if they could.
This leads me to the second reason why Yawgmoth's Will should be banned:
(2) The Development Trajectory of Vintage has Often Been a Race to Maximize Abuse of Yawgmoth's Will
The first deck to abuse Yawgmoth's Will that I witnessed in Type One was Keeper and Trix. Both decks fueled large game winning Yawgmoth's Wills. Advancements in Type One made Yawgmoth's Will more central.
Taking a look back for the moment, the Gush engine was very broken in GroAtog because the deck was naturally very powerful (the Gro base), and because of the interaction with Fastbond. But what made the deck ungodly was the combo that occurred when Yawgmoth's Will was cast. In the turns preceding Yawgmoth's Will, one Gush may have been cast from hand, and a Merchant Scroll might have found another, and a cantrip or two might have found the third. This was enough to maintain a solid advantage on the board and make Dryads and Togs quite formidable.
When Yawgmoth's Will was cast, things quickly spiraled out of control. In that turn - often turn 3 - in the general game and in the usual case at least four Gushes were cast (generating mana with Fastbond) along with Ancestral Recall and Time Walk, and Dryads and Togs grew to enormous proportions - often around 20 power. The Gushes helped find Yawgmoth's Will and Yawgmoth's Will made the Gush engine more than just a draw engine that fueled Togs and Dryads, it turned it into a combo deck that didn't even need Berserk.
The restriction of Gush helped the format slow down a bit, but the next deck upped the ante. The next deck to really abuse Yawgmoth's Will went beyond GroAtog and was really nothing less than a Yawgmoth's Will deck: Long.dec. Using Burning Wish and Lion's Eye Diamonds it found that 4 Burning Wishes and Lion's Eye Diamond synergy effectively enabled turn 1 or 2, game winning Yawgmoth's Wills with great consistency. For the first time in many years, two cards were restricted (Burning Wish and Lion's Eye Diamond) to stop that nonsense. This deck moved the Yawgmoth's Will turn from turn 3 or 4, to turn 1 or 2 - and made it possible by putting Will in the sideboard and finding it with Burning Wish.
Last year, my team developed Control decks whose entire strategy was to just draw cards and then play Yawgmoth's Will. Our Psychatog deck first made great use of this with the inherent synergy of Intuition. We one-uped ourselves with the Goth Slaver lists that used Intuition as well as Thirst for Knowledge to really make Yawgmoth's Will even more powerful and faster.
And now storm clouds are once again on the horizon. Gifts Ungiven has provided the perfect vehicle for truly abusing Yawgmoth's Will in fine fashion. Recoup is being played solely to flashback Yawgmoth's Will. My Gifts list - Meandeck Gifts is nothing less than an attempt to truly harness and concentrate the power of Yawgmoth's Will in a way that even our Goth Slaver list was unable to do. My Gifts for Time Walk, Tinker, Yawgmoth's Will and Recoup is a win-win Gifts. If they give me Tinker and Time Walk, I win on the spot. Anything else and Yawgmoth's Will costs less than seven mana. Yawgmoth's Will is so good that Meandeath is willing to pay half your life and seven mana just to cast Yawgmoth's Will.
This all relates to my third point:
(3) Yawgmoth's Will is Inevitably Going to Cause More Restrictions
I think the case for banning Yawgmoth's Will is very strong once the realization is made that future restrictions will have to be made entirely or partly because of Yawgmoth's Will. The pressure is building for cards like Dark Ritual and more pressing, Grim Tutor. That pressure would evaporate with the banning of Yawgmoth's Will.
This is sort of like the old Tolarian Academy argument: Ban Academy and you can unrestrict other cards. That argument never really held much water. You couldn't really unrestrict anything that isn't otherwise unrestrictable (at least you can't now). This is much worse though - because development in Vintage is often a battle to abuse Yawgmoth's Will, it will inevitably cause more restrictions. The most likely card to be restricted is Grim Tutor. I think it would be a big mistake until it proved dominant (because I think the deck is fair), people are already grumbling. The other card that could potentially be restricted at some point because of Yawgmoth's Will is Gifts Ungiven. With the legalization of Portal and the influx of more tutors, finding and playing Yawgmoth's Will can only become easier. Let's save ourselves the pain and suffering in advance. Other cards will be restricted because of Yawgmoth's Will. There is no other card that I can so safely claim that about. Yawgmoth's Will probably should have been banned back in December of 2003 instead of Burning Wish being restricted.
(4) Yawgmoth's Will Short Circuits Strategy or worse, Substitutes it.
I think this one is the most damning arguments from a mechanical point of view. Magic is a strategy game. Players have a general strategy for winning any given game. In some cases that just will be as mundane as attacking with creatures. In others, it is some victory condition such as Illusion of Grandeur + Donate. Whatever the case may be, in Vintage, far too many strategies strategy that a deck designer would try to build a deck around are inferior to just building your deck around Yawgmoth's Will first and foremost. Just as an example: Why should I build a deck around Psychatog + Cunning Wish for Berserk when I could build my deck around Yawgmoth's Will first? If I play Yawgmoth's Will, then I will have plenty of cards to feed to Psychatog and a million counterspells to backup my Berserk. For example, let's say I am holding Cunning Wish in my hand and I have a nearly lethal Psychatog on the table as a result. I can swing for a turn or so and then blow my hand and graveyard to just kill my opponent. Instead, I am fortunate enough to be holding Yawgmoth's Will. I play Yawgmoth's Will and replay my Accumulated Knowledges for 4 and 3 and Time Walk and Ancestral Recall. Now I can untap and kill my opponent with plenty of countermagic back up and the Berserk is really a formality so I can trample over my opponent's Mishra's Factory. The same goes for Gifts Ungiven. Meandeck Gifts doesn't really have a game plan outside of Yawgmoth's Will aside from Tinker + Time Walk. And Tinker + Time Walk is only so powerful because of the presence of Yawgmoth's Will in the deck.
My point is that a world without Yawgmoth's Will would be a far more interesting world because decks would actually have to struggle to execute their strategies - not play them out by proxy of Yawgmoth's Will. Win conditions don't even have to be powerful if the deck sufficiently abuses Yawgmoth's Will. If your Yawgmoth's Will is sufficiently broken, cards as slow as Morphling can finish the job because you have acquired enormous and overwhelming card advantage.
Good Type One players will often Duress a Yawgmoth's Will far in advance of a likely resolution simply because it is so threatening. It ends the game more quickly than the game would naturally have ended and helps reinforce the perceptions about Type One being less "interactive" than other formats. When Tog plays its second Intuition for Black Lotus, Mana Crypt, and Time Walk simply because it has Yawgmoth's Will in hand, you know that Yawgmoth's Will is a focal point of the deck.
It's not only a safe move to ban Yawgmoth's Will, it would be a wise move. I have no doubt that Yawgmoth's Will constraints deck design unnecessarily, and possibly even limits card design. Yawgmoth's Will forces the opponent to watch in resignation as all the spells that led up to Yawgmoth's Will over the course of the entire game are replayed in one turn because of one card. In other words, one player gets to replay the entire game from start to finish. If the cards in Type One are considered "accidents" or even overpowered, then all those broken cards that preceded the Will and made it more likely to show up imbalance the game all over again.
One final factor to emphasize is the universality of the card. Few decks cannot use Yawgmoth's Will and the splash for Black that occurs just for Will and possibly Demonic Tutor has so little drawback because of Fetchlands that the burden of proof is on the person who failed to include Will to explain why. It is useful in combo decks, control decks, aggro-control decks, prison decks, and aggro decks alike. Most restricted cards can't boast such universality. Necropotence is something control decks generally don't play.
I know that there will be resistance to what I am saying - regardless of its truth. The counter-arguments seem to fall into one of two categories.
Claim One: Yawgmoth's Will is just Another Broken Card and a Critical Part of Vintage
Part of the allure of Vintage is its enormous card pool, strategic diversity, and its brokenness. The top decks in Vintage are brutal. They are fast, relentless, and broken. But there is a broken balance in the format that makes it worthwhile. Decks go at each other and the struggle to pick up the bits and try and get your game plan together. Yawgmoth's Will is not just another card.
The closest card to Yawgmoth's Will in terms of game-winning power that can be played in most decks is Tinker. The only other two or three cards that I would even put on that level of power in terms of effect are Yawgmoth's Bargain, Mind's Desire, and Necropotence. But those three cards are very limited in terms of who can use them. And as for comparing Yawgmoth's Will to Tinker let me put it this way - Tinker is no Yawgmoth's Will. Yawgmoth's Will stands head and shoulders above all the other cards in the format in terms of sealing games. Zvi suggested that Tinker should arguably be banned. Turn 1 Tinker isn't really that dangerous for the format. First of all, turn 1 Tinker is most likely going to find Darksteel Colossus. That play is actually a goldfish turn slower than winning with Mox, Forbidden Orchard, Oath of Druids and Oath of Druids and Forbidden Orchards are unrestricted. Tinker is quite insane, especially since Mirrodin block, but it doesn't deserve banning because it doesn't win the game on the spot. Restriction actually works with Tinker. Only being able to run one makes the card much less powerful.
Claim 2: Banning Yawg Will Opens the Floodgates for Banning
This is probably the argument that causes the most concern. It is not warranted, and here is how you get around it:
There is no card in the format or in the game even remotely like Yawgmoth's Will. The more powerful a card is, the more you want to see it immediately. If Contract From Below (arguably the strongest Magic card ever printed) were Vintage legal, it would be a really broken turn 1 play. Yawgmoth's Will is not a broken turn 1 play. There are very few cards that have that quality and none on the scale of Yawgmoth's Will. As a result, restrictions will always serve to neuter the power of cards that people want to see on turn one. Restrictions will not just neuter the decks, but as I have shown, it makes the utility of the individual card itself drop.
Instead of opening the floodgates, banning Yawgmoth's Will can serve to uphold the principals that Vintage serves. It can stand alone in contradistinction guarding vigilantly against the claim that someone someday might put forward that some lesser devil should join it on the banned list. A simple comparison to Yawgmoth's Will will reveal the folly of such a claim. We should put it on the pedestal that says: This card is the best card ever. Banning Yawgmoth's Will can serve as a monument to enshrine the principal that in Vintage you can play with all of your cards and remind that banning need never happen again. I can think of no card more deserving of that honor.