Ari and Shaun have already written some fairly comprehensive overviews of the highlights of what we've seen of Fate Reforged, so rather than retreading what they've already covered, I'd thought I'd talk a little more about the implications of these cards and how they'll affect Standard. I don't like to focus on deckbuilding before we've seen the full spoiler, as fine-tuning is so important and requires the full card pool, but we can predict what kinds of decks cards will go into, and how those will line up compared to the known metagame.
First off, I think there's a good chance Ugin, the Spirit Dragon, is one of the best and most influential cards in the set. If that's so, Shaun's statement could accurately predict a radical metagame shift: "Such a powerful card could polarize the metagame and potentially turn Standard on its side by forcing all the midrange decks into hiding, since that is what Ugin is best against. We could see a world where control and aggro are actually the top dogs once again."
In recent months, we've been playing a Standard format where almost everything can be described as some kind of midrange deck, with the exceptions living on the fringe, though occasionally finding some success, as seen in Grand Prix Denver.
It's easy to imagine that all the Abzan planeswalker and Sultai or Abzan Whip of Erebos decks are comically horrible against Ugin, the Spirit Dragon decks. Those decks don't end the game quickly, they just build very powerful boards full of cards Ugin doesn't care about at all. Yes, they have enough card advantage that they can try to rebuild, and yes, they can Utter End Ugin, but it's not like the Ugin player is doing nothing else. When so much hard work can be undone so easily, you really have to wonder what the point is. Elspeth, Sun's Champion, Hornet Queen, and Whip of Erebos aren't top dog anymore.
The thing about Ugin is, it's an eight-mana spell. You can just make your midrange deck go a little higher on the curve, but let's be realistic, eight is a lot of mana. Elspeth, Sun's Champion can come down in time to stop some aggro decks as long as you can interact a little on the way. Hornet Queen at seven is a bit of a stretch for Abzan. At some point, you reach a cost that just isn't realistic. Unless you enter a board stall with a Courser of Kruphix in play, you can't count on hitting eight lands in a timely fashion in your midrange deck. If you want eight mana, you need to either play a lot of cards that give you mana (ramp), or play a lot of card draw (control).
Ugin is not just one step bigger in the midrange arms race. It could potentially be used that way in sideboarding in a midrange mirror, like we've occasionally seen with Garruk, Apex Predator, but in practice, Ugin is going to go in new decks built around it.
Shaun suggests a handful of cards that are going to be good against Ugin, but he ignores the important ones. The only really good cards against Ugin, the Spirit Dragon are counterspells. You don't have to worry about leaving mana untapped at that point in the game, you'll just have mana lying around, and if you have a counterspell in hand, Ugin's not entering the battlefield.
I can imagine a world where Ugin, the Spirit Dragon doesn't impact the format much. Fate Reforged looks to be an extremely powerful set, and a lot of the power is in aggressive creatures. If the aggro decks are good, and the midrange decks include blue for Disdainful Stroke, Ugin might just not be the finisher for the job. If the world continues to look like it has, Ugin's the real deal.
Decks that use Ugin, the Spirit Dragon could look something like the Green Devotion decks that we've seen, ramping up to the powerful planeswalker with colored permanents, all of which will be exiled if it uses its -X ability, on the theory that if it comes down early enough, you might be able to get away with just using the +2, or, barring that, once you've killed everything but your Ugin, Ugin will simply be enough.
Another familiar-looking build would be something close to Andrew Brown's deck from Denver, U/B Control with 28 lands and a lot of card draw, making it easy to hit eight mana on turn eight.
We could also see more unusual takes. Font of Fertility, Frontier Siege, and Nissa's Expedition offer green non-creature-based ramp. For that matter, Nissa, Worldwaker herself works extremely well with Ugin, the Spirit Dragon, as the animated lands don't get exiled, Nissa doesn't get exiled if X can be four or less, and a turn 4 or 5 Nissa allows you to cast Ugin the next turn by untapping forests if you have three or four. We could easily see a green ramp deck that's built to be minimally hurt by Ugin's second ability that would love very different from Green Devotion decks we've seen. Xenagos, the Reveler is another good option for quickly ramping into Ugin, the Spirit Dragon.
Finally, we could get a little more creative with our Ugin ramp. Maybe we just play some banners in a control deck, or maybe we try to spike it off Narset, Enlightened Master.
First of all, if all you do is play a tapped land, then a Satyr Wayfinder, you can cast this on turn three. That's a 4/5 to start beating down, or it can just hold the fort and start drawing cards. I think Ari and Shaun radically overstate the disadvantage of your opponent getting to choose which card you get. I think this ability will generally be better than four mana: draw a card.
I think Tasigur will be at its best in controlling Sultai decks with Satyr Wayfinder to power it and a lot of instant speed counterspells and removal. Basically, you want to be able to pass with mana up and activate Tasigur if you don't have to do anything else. I think it's realistic to build a "protect the queen" control deck around Tasigur, since you can play it while holding mana up to protect it in the midgame and it's big enough to stabilize a board. Finding the exact balance of other delve spells will be tricky, but you'll definitely want more than just Tasigur in this kind of deck.
It will also be outstanding as a 1- or 2-of in basically any Abzan deck. So often, the game goes long and comes down to an attrition battle, and it'll be easy to play Tasigur as your last card and activate it immediately, getting ahead even if they have an answer, and essentially winning the game on the spot if they don't.
When the game isn't an attrition battle, everything trades and graveyards fill quickly, and the best way to win is to play two spells in a turn. Tasigur is perfect for this kind of game as well, as you'll be able to play it and another threat or answer on around turn 5, in plenty of time to swing the tempo of the game in your favor.
Finally, Tasigur is also great in U/B Control. It competes with Dig through Time and Murderous Cut, but I think it competes well enough to earn a slot or two. Here, it plays very similarly to how I described in the Sultai deck, but you lose the opportunity to slam it on turn 3 off a Wayfinder and get aggressive.
Hooting Mandrils has been seeing some play in Modern and Legacy, and this card is much more powerful, perfectly positioned in Standard, and the right size to dodge Stoke the Flames and block almost every creature on the ground.
Just as Tasigur stole my attention from Ugin when reading the spoiler, I think Tasigur may be the card that keeps Ugin out of Standard, by creating another great home for counterspells and a different way to dominate late games.
While these are the great new control and midrange trumps, Fate Reforged also does a lot to push aggressive decks. Yasova Dragonclaw, Shaman of the Great Hunt, and Flamewake Phoenix are poised to push Ferocious as a mechanic beyond what we've seen so far. Yasova offers another huge payoff for Elvish Mystic on turn 1, something G/R was desperately looking for. Fanatic is Xenagos might not be embarrassing, but I think Yasova is a substantial upgrade, especially if you're trying to make Ferocious matter.
Flamewake Phoenix is the most exciting of these to me. It curves perfectly into Ashcloud Phoenix and Stormbreath Dragon, offering an impressive "Big Red" Skies strategy, or it could top off a red aggressive deck pushing haste with Shaman of the Great Hunt replacing Ashcloud Phoenix.
Shaman of the Great Hunt didn't impress me on first reading. It's much less aggressive than something like Hero of Oxid Ridge, but it has the advantage of being a perfect follow-up to all three of red's best three-drops. Goblin Rabblemaster and Hordeling Outburst are both likely to leave you with multiple creature that can get through by going wide. You might lose one, but the others will grow (assuming they didn't kill your Goblin Rabblemaster) and Flamewake Phoenix's evasion means that it will quickly grow to the point where your opponent will have to deal with it, which won't generally be a particularly effective use of a card (though, sadly, the counter will put it in range of Abzan Charm, but really, Shaman of the Great Hunt will be the first priority, and your Phoenix will continue punishing them).
On the subject of sacrificing creatures, I would be remiss if I ignored Outpost Siege, the successor to Boggart Shenanigans. Boggart Shenanigans is a bad and rarely played card that I used to win a car in 2007. While Boggart Shenanigans cost one less mana, it was a lot less versatile. It only worked with Goblins, it only hit players, and it didn't have a second mode that does something completely different.
Lately, I've been playing a R/W Aggro deck that's built almost entirely around using Chandra, Pyromaster as a card advantage engine. Outpost Siege is somewhere between "interesting" and "literally perfect" for a deck like that. When I have a hand with a lot of removal and the game becomes a grind, it's a Chandra. When I have a token draw and I start attacking, it's a Boggart Shenanigans. Basically, this is the Purphoros, God of the Forge that all the tokens decks actually wanted. Purphoros is tough, because it's a four-mana spell that doesn't impact the board the turn you play it that you want to play before you play your creatures. Outpost Siege is also a four-mana card that "doesn't impact the board," but you want to play it after you play your creatures, and there's a good chance it will immediately impact the board if you have creatures, as they'll likely die before you untap, and Outpost Siege will let them take other creatures down with them.
The two modes complement each other perfectly, despite looking totally disconnected on the surface, and there's a good chance that this card is a lot better than it looks. The only thing really holding it back it that it's a four-mana enchantment at a time when people actually play enchantment removal, but I don't think that's a dealbreaker.
Finally, we have the pair of white mythic creatures that work well with spells, Monastery Mentor and Soulfire Grand Master. Both of these are potentially good in Jeskai Token/Ascendancy-style decks or could work just as well in something like my R/W deck.
I agree with Ari that Soulfire Grand Master is likely slightly overhyped at the moment. It is better in less aggressive decks, and most of the decks that want a white two drop will rarely spend a lot of time with six mana when they can use this guy, and it'll often die before it matters. Ajani's Sunstriker isn't particularly close to playable in Constructed. That said, I think the interaction with Stoke the Flames is legitimately great, and that by itself makes it worth trying. I would note specifically that this isn't a card I'd expect to want to play four of, as it doesn't do anything extra in multiples.
As for Monastery Mentor, yes, that's a lot of explosive power, but in Standard, how are you really taking advantage of it? I've tried to build the Dragon Mantle/Defiant Strike decks, and I'm pretty sure that's not the way to go. What are the cheap spells we're casting to take advantage of this? I think the answer starts with Valorous Stance, Gods Willing, and/or Ephemeral Shields, which pleases me, as I like Ephemeral Shields a lot as a card, but I'm not totally sure that's enough. It's possible that this will be best as an extremely unconventional looking finisher in a control deck--something we plan to cast on turn 5+ to protect as we start taking the game over, but will that really be better than just casting some dragon, planeswalker or Pearl Lake Ancient? It's a powerful card, but I could easily see it falling well short of the hype I've seen so far.
There are many important utility type cards that I haven't really talked about, like Wild Slash, Reality Shift, and Valorous Stance, but those are the exact kinds of cards I referred to at the beginning--the cards that go into fine-tuning a list, where we'll need to see the full set to even begin to get a picture of what the metagame will look like to allow us to start choosing the right mix of utility spells.
For now, it's the higher-impact build-around cards that we can take some time to mull over. I personally can't wait to start Tasiguring people, despite all the reasonable additions to the deck I've been playing.