This is a topic that gets covered fairly often, but it's also one that's valuable enough to stay updated on. I was happy with my take away article from Theros Block Constructed, and I think it's worth trying to break other formats down to understand the driving forces to conclude where a format should naturally lead. This is particularly important in formats that feel solved or stale, as this Standard format seems close to.
Let's start here:
Brian Kibler (@bmkibler) June 2, 2014
Thoughtseize is an incredibly powerful card. The biggest impact of Thoughtseize is to punish decks that rely on a single card (most notably Sphinx's Revelation), but in general, it weakens any dedicated card you can play against a strategy that uses Thoughtseize, and to some extent broadly encourages redundancy and aggression. Thoughtseize has also made black possibly the most played color after a long stint in Standard where it had been the least played by a considerable margin. Every deck needs to expect to start many of its games by being targeted by a Thoughtseize.
Whether reprinting Thoughtseize was a mistake or not, Magic's developers knew that they were releasing an incredibly powerful predator into Standard and built tools into the format that could help withstand it.
The best way to beat a Thoughtseize has always been to have your best card on top of your deck. Sensei's Divining Top has been among the best tools against Thoughtseize in Legacy, and while we can't have anything that powerful, we have been given a cycle of lands that help manipulate our draws.
This brings me to the next pillar of Standard - the temples.
When Theros came out, I wasn't terribly impressed by the temples; I recognized that they were a relatively powerful dual land, but I didn't think there was enough of a reason to be multicolor to care about a good dual land. Now that a three-color deck can play twelve temples, I've changed my mind on the subject substantially.
I currently believe (with admittedly low confidence) that conventional wisdom is leading the hivemind to misbuild their Standard decks. In Theros Block Constructed, our two-color decks would usually play some number of off-color temples just to have more scrying. Most three-color Standard decks start with 11-12 Ravnica Dual lands, and then add 6-8 Temples. I believe this is backwards. Unless your deck is a dedicated aggro deck that is actively looking to pay life to curve out, it's worth playing the maximum number of temples. In addition to this, I think people are playing too few lands. It's important to draw lands early so that you can play the game, hit your curve, and sequence your spells right, but after the beginning of the game, you don't want to draw many lands. I'd like to play enough lands that I can always look at my opening hand and know that I'll be able to play my spells, and then use my scry lands to get rid of extra lands. I could play fewer lands and use scry lands to find more, but then I'm keeping more speculative hands, and I have less control over exactly when I play my untapped lands. I think the way to build midrange and control decks in Standard is to play 12 temples, 25-27 lands, and try to play as many basics over Ravnica lands as possible.
This configuration results in paying less life (so you're better against burn), getting more card selection (so you're better against Thoughtseize), having more lands (so you mulligan less and curve out more), but more scrying (so you flood less later). It's just the best of all possible worlds.
The temples are amazing, and radically underutilized in Standard at the moment. Having ported a Theros Block Constructed basis for a mana base over to Standard, I'm loving how it plays out, and feel cheated any time I have an opening hand that doesn't have 1-2 temples.
The other important land in Standard is Mutavault, which I consider the most powerful card in Standard.
It's extremely important in Magic to play lands that you can get value out of. When Theros first came out, I thought Mutavault was an absolute requirement for any Standard deck. Now that a three-color deck can play twelve temples, which offers another way to get value out of lands, I think it's more negotiable, but Mutavault remains extremely influential. It's the last remaining reason that I would want to play a mono-colored or aggressive deck. I currently think the advantage of playing twelves temples is strong enough that I'm somewhat disinclined to build around Mutavault, and the prevalence of Sylvan Caryatid and Courser of Kruphix further punish Mutavault.
Mutavault is the only reason to play Black Devotion without Courser of Kruphix, and most of the reason burn can compete at all (though I think that should happen less as people move away from Ravnica lands). Mutavault is also a substantial contributing factor to why the green creatures that block it are so good.
Courser of Kruphix is the next major pillar of the format. This card is so much more significant than it might first look. It's an absurdly oppressive card for aggressive decks to try to beat. It has the perfect numbers to brick almost any early ground offense, and the lifegain it offers radically punishes strategies that are based on smaller evasive creatures or burn. The value that it generates against midrange and control decks is strong enough to round the card out so that it's good against everyone, which means aggressive decks have to fight a card that's powerful enough to feel like a sideboard card even in Game 1. It plays excellently with temples, allowing you to scry away lands that you would draw so that you can keep your draw step restricted to spells while still playing lands from your library, and also benefits from my suggestion to play more lands. At face value, the card is amazing, but it gets even better when combined with anything that lets you draw cards during your main phase, or any nonland that lets you scry, so that you can ensure that you'll never miss a land or a life.
The impact of Courser of Kruphix is to push everyone toward midrange. You don't want to play small creatures that will just get bricked, but you also don't really want to play creatureless (Why wouldn't you want your own Courser of Kruphix?). It makes games go long, and it makes the ground lock up. It's great with most planeswalkers, so it pushes toward relying on Planeswalkers and fliers to win games.
As implied but not directly stated, I basically think trying to get under Courser of Kruphix is a lost cause. If you want to attack, you should have a plan to go over or through it. Throwing more undercosted 2/1s and 2/2s at it just won't get you anywhere.
Sphinx's Revelation has to be mentioned among the pillars of the format. Thoughtseize attacks it, but if every game is going long, and players are playing a lot of temples, you can always plan to find it later and cast it to great effect. If the world is defined by Courser of Kruphix, Sphinx's Revelation should be extremely well-positioned to take advantage, as it's a way to go over the top of the value gained by Courser of Kruphix, and if you have your own, it benefits strongly from missing fewer land drops. I haven't been seeing a lot of good results from Sphinx's Revelation decks recently, and I think that's because of the combined pressures of Thoughtseize and planeswalkers, but it still feels like the card should be able to overcome those pressures in the right deck.
Underworld Connections has been the unsung hero of Black Devotion for its entire lifespan. Some W/B decks have replaced it with Read the Bones to quickly curve up to Elspeth, Sun's Champion and try to win the game quickly, but ultimately, Ii's Underworld Connections that has kept this deck at the top. Thoughtseize and Mutavault gave it the power it needed, but Underworld Connections was how it was actually able to grind out wins against a control deck, or how filling a deck with removal could ever seem like a winning game plan. As the format slows down and playing decks that have multiple colored mana symbols in multiple colors becomes more realistic, I think we'll see Underworld Connections and Courser of Kruphix as one of the central interactions of the format. Each card is great on its own, and both have seen success without the other, but the power of both combined in radically greater than the mere sum of each card individually.
These are the key driving forces that I see moving forward. To recap:
There are a lot of other cards that I see as extremely important but either more niche or more replaceable. Thassa or Master of Waves, for example, would be profoundly important if I still had faith that Mono-Blue Devotion was a major player in Standard. The fact of the matter is that I don't think it is anymore, and even if it was, it's just one deck; the cards above are cornerstones that can be used in a variety of ways.
Also, I'm not claiming that these are the most played cards. Hero's Downfall, in particular, is another card that sees a tremendous amount of play, and it's extremely influential (honestly, it almost makes this list). The reason it's not quite on the level of these other cards for me is that I think it's more replaceable. Yes, it answers everything in a way other cards don't, but I think most of the deck could get by with other removal spells, or at least, the versatile nature of their removal isn't the defining characteristic of the deck that way that these other cards are.
There are a number of planeswalkers that are hugely significant. Domri Rade; Xenagos, the Reveler; and Elspeth, Sun's Champion, in particular, though there are others. Each one individually doesn't seem all that significant to me. There are a variety of planeswalkers, and all of them do what planeswalkers do. You need to be able to kill them or they will beat you. This is likely just a reality of Magic these days: if every set has 1-5 planeswalkers, and every planeswalker is anywhere near the power level we're used to, a substantial portion of decks will be built to take advantage of one planeswalker or another, and any of them left unchecked will usually win.
Pack Rat is a card some might point to as a possible pillar of the format. It got a lot of press for being oppressive out of Mono-Black Devotion, but while it's quite powerful, I believe that it's ultimately replaceable. The Pack Rat decks benefit from some number of free wins off unanswered Pack Rats, but that's not the core strategy of the deck, which is functional without it.
Nykthos, Shrine to Nyx likely was a pillar of Standard previously, but as the completion of the cycle of temples has opened the door to more and better multicolor decks, the incentive to build around Nykthos has decreased. Further, I think we're at a point where people are playing a lot of removal regardless of whether Nykthos is a relevant concern, which mitigates its power level. While there are decks that exploit the card very effectively, I think they're still too niche for me to rank it among the others.
Scavenging Ooze, Stormbreath Dragon, and maybe even Nightveil Specter still--but the case is getting weaker for that--join Pack Rat as being exceptionally powerful threats, and if you're interested in trying to use your graveyard, you absolutely need to respect Scavenging Ooze in particular, however, like Pack Rat and planeswalkers, I think these threats are just too replaceable.
I haven't mentioned any card in R/W Burn except for Mutavault. This isn't just because I have a bias against decks like that. First, they're all narrow cards. Second, it's basically a metagame deck that looks to punish specific strategies. And third, each card in the deck is relatively replaceable. The entire point of the deck is its redundancy that all the cards are basically the same.
There are some other relatively powerful and relatively unique effects that are extremely important to be aware of in the format. Cards like Supreme Verdict and Rakdos's Return are excellent ways to punish certain strategies and cards you need to be deeply aware of walking into, but again, those are just a little too narrow to really be viewed as the driving forces at the moment.
Finally, the last card that didn't make my list--but easily could have--is Sylvan Caryatid. I basically think this card has all the characteristics needed to make the list, but I think it's fairly redundant with Courser of Kruphix. I think playing one without the other is extremely rare, and they both push in exactly the same direction.
So what does all this mean?
I think I've sprinkled that throughout the article. My current belief is that three-color midrange centered on black and green is the default best place to be. I think there are far more realistic alternatives than there were in Theros Block Constructed, but that's the starting point. There are valid reasons to play each of the color colors. I'd likely try to pivot around either of those colors, but I still haven't played enough to confidently conclude that any of the other strategies we've seen function previously aren't viable now.
Play more Temples!