Note: I write set reviews from a tournament player's perspective, not a financial perspective.
As such, I try to approach new set releases with a "wide net" of all the cards which might be relevant to tournament Magic, even if it's just a sideboard 1-of that will never be worth much. As such, this will necessarily include fringe cards. For commons and uncommons, I'm not going to talk about every single card, since a lot of them are obviously Limited-only stuff.
I strongly recommend buying a complete common/uncommon set for all expansions, since that can be had for around $20-40 (depending on whether you're buying locally or online), but the core set is full of a great many reprints (and many of the new cards are going to be purely Limited fodder), making that a waste of money for all but the newest players.
Also, card tags will be broken on new cards until they're in our WordPress plugin. However, they're included for the people who refer back to this article.
For those of you reading before card tags are working, follow along with a spoiler (like this one) and let's explore the new possibilities M12 brings.
Celestial Purge: This is a relevant sideboard reprint, though the question is whether the versatility here is enough to offset Leyline of Sanctity's ability to outright shut burn down. I'm guessing not, and at least where red is concerned, Kor Firewalker is a better option. It is a good answer to Bloodghast, so it'll remain in the mix for at least that long. This is a solid card which should continue to remain in the core set - it gets color hosing right.
Demystify: Another reprint, and this has the potential for fringe usability against Splinter Twin - one that can get past the tapped land from Deceiver Exarch a bit more easily. If there are other enchantment-based decks around, it's at least worth remembering that this is an option.
Elite Vanguard: Sadly, white weenie hasn't really cut it since Ranger of Eos rotated out of Standard, but with Honor of the Pure in the format it's worth remembering that dropping 2 power on the board on the first turn is never a terrible idea. Right now, Goblin Guide makes this guy look silly, but that's not always going to be the case. Puresteel Paladin may need some raw bodies to slap equipment on, and this guy isn't a bad choice.
Oblivion Ring: Welcome back. You never should have left. The important thing here is that Leonin Relic-Warder and Kor Sanctifiers just got better again for a while. The other thing to remember is that when O-Ringing to kill a Planeswalker or a Legendary permanent in a mirror match, you should exile their O-Ring (bringing yours back and letting the Legend rule solve problems) instead of their Planeswalker. If you remove the Planeswalker directly and they kill the O-Ring, you're put in a very bad position.
Timely Reinforcements (NEW): I've seen some people claim that this is the nail in the coffin for Red. I disagree. It's obviously a bad card for Red to deal with, but it's not so bad as to be automatic game over. With Shrine of Burning Rage, red tends to get into overkill territory pretty easily, and the creatures off the Reinforcements will find themselves ready targets for Arc Trail, Searing Blaze, and Grim Lavamancer. One important thing to note is that Timely Reinforcements pushes white players more towards Leyline of Sanctity than it does towards Kor Firewalker.
With use of fetchlands, white decks can maindeck this if they want, but it's iffy against combo decks like Valakut, Exarch-Twin, and Quicksilver Amulet. If you're not getting creatures off the Reinforcements, it's a bad card.
Aether Adept: Will continue to not see significant play, much to the continued annoyance of those who played Man-o'-War back in the day. However, it could have a home as a 1-of in a Birthing Pod deck or something similar.
Azure Mage (NEW): This is an upside to the blue half of Dimir Guildmage, since the card drawing can be played at instant speed. That's huge, and likely hurts Druidic Satchel's chances of seeing play by a lot. The question is whether blue would rather pay mana over time to draw extra cards, or get a fatty as part of the deal with Consecrated Sphinx. It's entirely possible that the answer may be a mix of both.
Divination: Not amazing, but typically just barely good enough. This might fall by the wayside in decks packing Azure Mage and Consecrated Sphinx, while decks that can gain value from proliferation will prefer the "colorless" Tezzeret's Gambit.
Flashfreeze: Constantly showing up in sideboards, it turns out that a 2-mana hard counter is often exactly what blue needs in certain matchups.
Mana Leak: This is such a strong card that it's hard to believe that people actually complained about Counterspell leaving. It's quite clear now that Mana Leak replacing Counterspell made deckbuilding skill more important since getting the right mix actually took a bit of effort. As a general-purpose counterspell, Mana Leak is right where it needs to be.
Master Thief (NEW): Possibly a fringe 1-of for Birthing Pod mirrors, since it lets you have an answer to Pod on the 4-spot of your curve without playing Red for Oxidda Scrapmelter. It could also serve as random utility in or against the Tezzeret, Agent of Bolas deck, if one exists. I don't think the metagame has what it takes for this to see widespread play.
Merfolk Looter: Once an all-star, this sat on the sidelines when it was in M10, and will likely sit on the sidelines in M12. Sadly, the talk of moving looting to red doesn't seem to have panned out. With Coralhelm Commander and Grand Architect, is there a deck for this to see play in? I doubt it. It sits alongside Enclave Cryptologist and Hedron Crab as possible ways of dumping Vengevine in the yard, but Fauna Shaman is just so much better at it that there's no reason to bother.
Mind Control: The ultimate answer to a single oversized creature, this card doesn't see much play, but can utterly blow people out in Constructed just as much as it does in Limited. The catch is that Corrupted Conscience turns most of the things you'd like to steal into one-hit kills, while Volition Reins can steal even more things, so there's not much room for Mind Control right now.
Phantasmal Bear (NEW): A 2/2 for 1 is worth considering, even if it is in blue. There may be some sort of tempo deck centered on this, Cosi's Trickster, Vapor Snag, and counterspells. I think it's unlikely, but if something odd happens to the metagame, it could work.
Phantasmal Dragon (NEW): A 5/5 flier for 4 is a respectable card even with the current level of power creep, but right now it dies to everything. Beast Within doesn't even have the dignity to give you a token when they kill it! You can't even counter their spell to save the Dragon, so it's probably not worth the hassle. Still, it's possible this could be part of a transformative beatdown sideboard for Exarch-Twin decks.
Ponder: Now this is a big deal. Some people are claiming that this card's return will single-handedly bring Pyromancer Ascension back to the forefront of the format. I am somewhat skeptical of Ascension's ability to compete with Exarch-Twin or Quicksilver Amulet, but people are clearly going to be trying it early. (If mill-based Ascension becomes popular, Amulet-Eldrazi shuts them out entirely.) It gives a boost to Exarch-Twin as well, since they gain more ways to dig for the combo, but blue control decks will probably stick to Preordain.
Turn to Frog (NEW): In order for this to see play, it has to be effectively a blue Doom Blade. Yanking a Baneslayer Angel out of the sky or turning Primeval Titan into a puddle of slime only to block with something - even a Calcite Snapper - accomplishes this goal. It's worse than Tumble Magnet in some situations, worse than Dismember in others, and in Blue-Black decks you may have access to actual Doom Blade and don't even need to consider this. This is also a 1-turn answer to Exarch-Twin. When they activate a Splinter Twin-enchanted Deceiver Exarch to make a copy of itself, you respond with Turn to Frog. A copy will still be made, and if they choose to untap the original Exarch, they're untapping an ability-less 1/1 Frog. If you think you want to play it before the enchantment resolves - big mistake. The ability-removing continuous effect and the Splinter Twin's ability-granting continuous effect are in the same layer, so whichever one has the later timestamp will win out.
Deathmark: This has mostly disappeared from the format largely due to metagame reasons. It's unlikely to return to regular play, but with the loss of Disfigure at rotation, people may want a cheap way to kill small green/white creatures that don't require paying 4 life.
Distress: Duress is gone at the rotation, which means Despise is the main 1-mana discard spell. Unfortunately, it's not particularly good against control decks. Distress is pricier but far more versatile. It won't be universally adopted, but some lists will run it.
Smallpox: Bloodghast players are going to have some fun with this one. Sadly, there's no Flagstones of Trokair to abuse right now, but even if you can get yourself up by 1 via Smallpox, it should be enough to give you a sizable lead.
Zombie Infestation: For you to want to play this card, you either need recursion, or a belief that the two cards you're discarding are actually worse than a pair of 2/2s. If you're building your deck right, the second part shouldn't be happening (but it's a nice bonus when you draw too many lands).
Circle of Flame (NEW): This is an awkward effect since Pyroclasm and Slagstorm often do a much better job of sweeping the board, but Pyroclasm will be gone. Multiple Circles stack, so it's not totally useless against decks with X/2s, but it's obviously a sideboard-only card.
Combust: As an uncounterable answer to Deceiver Exarch, Combust has a home for at least a little while. Everywhere else, it's a bit awkward - it can't kill Kor Firewalker, it doesn't kill Titans, and it doesn't kill Consecrated Sphinx. When Baneslayer Angel was around, Combust was a lot better. If white and blue creatures with less than 6 toughness start to need killing, this will remain Red's best way of doing so.
Goblin Fireslinger (NEW): This is an excellent way to trigger Bloodthirst, but it doesn't look like that will matter in Constructed. More importantly, it's a guaranteed 1 damage per turn this guy stays alive, no matter how clogged up the board gets. That may be worth a spot.
Goblin Grenade: If Goblins is a real deck, this is the reason why. Being able to Lava Axe someone out from a single mana is incredibly potent. If the need arises, it can kill a creature to clear the way. Goblin Arsonist is obviously the fodder of choice, but nobody will mourn the loss of a second Spikeshot Elder or something along those lines. The only issue is that the reason to play Goblins over conventional RDW is reduced vulnerability to Leyline of Sanctity, and Goblin Grenade has that problem. Of course, you can simply board it out in games 2 and 3 if that is a real issue. Leyline also rotates out with Innistrad's release, which may also be a point in favor of conventional RDW at that time.
Incinerate: While this is obviously worse than Lightning Bolt, Red players don't have much room to complain. For a few glorious months, both cards will be in the format together, alongside both old and new Red all-stars Grim Lavamancer and Goblin Guide. The no-regeneration clause would be more appreciated if Thrun, the Last Troll didn't have Hexproof or a 4th point of toughness.
Manic Vandal: An underappreciated utility dork, Vandal gets the job done. While I'd much rather have Smash to Smithereens, you take what you can get, and a Gray Ogre that takes out artifacts and gets in there is just fine. It's one of those cards that's never exciting, but you're always happy to have around.
Autumn's Veil: While Summoning Trap was around, people used that to punish those who would dare to counter their creatures. Once Trap is gone, something like this can be used to guarantee that the creature resolves- and makes them waste their Counterspell or kill spell in the process. With the popularity of Dismember, this is an excellent tool for green to have available.
Jade Mage (NEW): There's actually precedent for cards like this seeing play: Selesnya Guildmage was used in Ghazi-Glare, a deck designed to use Glare of Subdual and a token army to suppress the opposition until Yosei, the Morning Star or the token army itself was able to close the game out. Obviously, Jade Mage doesn't have the ability to pump the entire swarm, but Eldrazi Monument, Beastmaster Ascension, Overrun, and Garruk Wildspeaker are all currently legal. Having the ability to channel extra mana into token generation is almost certainly worth a slot in a Fauna Shaman or Green Sun's Zenith deck.
Llanowar Elves: A longtime staple among the little green dudes, Llanowar Elves loses his buddies Joraga Treespeaker and Arbor Elf on the rotation. This may be a problem, but even if Elves isn't a deck, Llanowar Elves and Birds of Paradise are among the few ways to have 3 mana on turn 2. If the format has lots of powerful 3-drops, this will continue to be relevant. If the key mana cost is 4 or 5, the vulnerability of being a creature will push people more in the direction of Rampant Growth and the like.
Naturalize: This has lost popularity because of Nature's Claim, Beast Within, and the ability to Pod or Zenith Acidic Slime. Still, it's one of the best cards for doing what it does, and it's always worth considering for sideboard slots.
Overrun: Apparently it's considered fair in Limited for a 5 mana sorcery that reads "win the game" to be printed at uncommon.
Rampant Growth: Providing a major boost to Valakut, Rampant Growth replaces Cultivate in M12. Accelerating from 2 to 4 mana on turn 3 is the primary use of this, but it can also serve to achieve a general mana advantage in general in something like a U/G archetype, even if it wants to leave Mana Leak open on turn 2.
Dragon's Claw: In formats with an extremely potent Red deck, the Claw has seen play before. Some Jund lists sideboarded it last year, and it's not unreasonable to try to play it now.
Buried Ruin (NEW): Academy Ruins this isn't, but this might be good enough anyway. Bringing back a Sword of War and Peace or Wurmcoil Engine that got killed is an obvious use, but recurring something as simple as a Nihil Spellbomb could be enough to put the game away. It puts it in your hand, unlike Academy Ruins, so trading the land for even a minimally-useful artifact can be worthwhile when you're manaflooded or a game goes long.