Every time a new Magic set comes out, everybody on the Internet is totally sure they totally know which cards are going to be overpowered and which will be completely unplayable.
It turns out, people are not really that amazing at predicting what's going to be good. Sometimes there are good excuses for these misjudgements - Melira was only a thing in Modern, which was just starting back then, Stoneforge Mystic didn't have any good targets at time of its printing, and hybrid mana cards like Burning-Tree Emissary wouldn't see as much play if it wasn't for unrevealed at a time devotion.
Still - a lot of these mistakes have no excuses. Titans in Magic 2011 and devotion in Theros have been horribly underestimated by just about everyone (and I'm showing only the most extreme examples below), even though all cards for Theros Standard were known, and Titans would have been very playable in just about any Standard.
The list below is provided not to shame anybody. It's just for your entertainment, and in hope people will have a bit more scepticism about their ability to predict how good cards will be without playing.
EDIT: Apparently Grave Exchange rating was supposed to be a joke, it's hard to tell among so many awfully missed reviews which are serious mistakes and which are jokes. Anyway, I removed it since there are enough unintentional failures to enjoy.
EDIT: A year later, I wrote followup post with more extremely missed ratings.
Worldwake - Stoneforge MysticConstructed: 2.0
This is unlikely to make a splash in Standard, but getting Umezawa's Jitte in Extended makes this quite interesting. I doubt that it makes something as clunky as Tatsumasa, the Dragon's Fang worth it, but even if it only can get Jitte, there are decks that are interested.
Magic 2011 - Frost TitanConstructed: 1.0
Would shroud really have been too much to ask for? As is, this Titan doesn't even net you a whole card when you play it, and its ability doesn't really protect it by the time you get around to playing it.
Magic 2011 - Sun TitanConstructed: 2.0
I don't have high hopes for this particular member of the band. He is doomed to play bass while Primeval Titan and Grave Titan fight over the mic, since getting a small permanent back is just not sexy enough. Yes, getting back O-Ring is pretty sweet, but how often does O-Ring actually hit the bin? Opposing O-Rings are the most likely answer, and Sun Titan doesn't help there. As for getting back Tectonic Edge or a fetchland"¦that isn't my idea of an exciting six-drop. Just play Sphinx of Jwar Isle or Baneslayer Angel at that point, since Sphinx dodges removal and Baneslayer flat wins the game if they don't have it. Sun Titan does neither.
Magic 2011 - Inferno TitanConstructed: 2.0
This passes the value test (if it dies immediately you still got value, in case that wasn't clear), and is a solid threat in its own right. The effect isn't as powerful as its undead cousin, and red isn't very well set up to play six-drops, but as a card it is good enough to consider. Finding a deck might be difficult for this, since it doesn't fit into any of the existing archetypes.
Scars of Mirrodin - Sword of Body and MindConstructed: 1.0
I was excited to find out that they were continuing the Sword of Fire and Ice cycle (sorry Light and Shadow, but that’s how it is), but my excitement dimmed as soon as I read Sword of Body and Mind. The abilities just don’t match up at all, and neither is particularly powerful. Milling them while bashing for 4+ damage doesn’t accomplish anything, and is actually more likely to be a drawback than anything else. Sending a bunch of Vengevines to their yard is not quite what I’m looking to be doing. Besides the useless mill ability, you get to make a singular Grizzly Bears, which is also not that great. I have higher hopes for the rest of the cycle, since this one just isn’t good enough.
New Phyrexia - Melira, Sylvok OutcastConstructed: 1.0
I don’t see this working out very well as a sideboard card. If it didn’t take away infect, it would be an awesome hoser for block, but playing a 2/2 for 2 that just gains you a bunch of life against one very specific deck isn’t going to cut it.
Innistrad - Delver of SecretsConstructed: 2.0
This might actually work, which is pretty sweet, since this is one of the many cards in the set with awesome flavor. Dropping one of these guys and Pondering should just about do it, and random flipping in a deck full of spells doesn’t seem out of the question. If there is an all-spell sort of deck, he could be a legitimate threat.
Innistrad - Skaab RuinatorConstructed: 3.5
This thing is gonna ruin some hopes and dreams. Coming out early and often doesn’t seem unrealistic at all, and there are even multiple ways to achieve this goal. The first is Birthing Pod, since he even only costs three, and once sacrificed, can be easily brought back. Multiple 5/6’s on turn four and five is tough to beat, and that’s without the backup plan of recasting them if the opponent manages to kill both all your guys and the Pod itself.
The second way to maul them involves milling yourself, probably with Forbidden Alchemy and Dream Twist, and likely alongside Unburial Rites and other sweet Flashback cards. This deck seems a bit more creature-light than the ‘Pod deck, but between Snapcaster Mage, Skaab Ruinator, and possibly even Armored Skaab, you have a good start.
Either way, I expect there to be much ruinating over the next year, and it should be pretty sweet.
Return to Ravnica - Ethereal ArmorConstructed: 1
If Enchantress decided they want to start winning on the back of their enchantress itself, this has a shot. The big problem there, is typically the only enchantress that sees play has Shroud (that's like a more fair version of Hexproof for all you young'n's), so there aren't actually any good targets for this.
Return to Ravnica - Pack RatConstructed: 2.0
PV (@PVDDR_Quotes on Twitter) compared this to Bitterblossom. Even though he was joking, given enough time, the opponent will be plagued with Rats in short order. If they don’t pack enough removal, you get more and more value with each activation, and the copies retain the activated ability in case of the original dying.
The power level here is interesting, and if there are enough attrition-based matchups, this could certainly be a thing.
Gatecrash - Burning-Tree EmissaryConstructed: 2
These types of cards are always begging to be broken. I am not sure if this one will end up being in that level of power, but rumors have already begun spreading about some of the things you can do with this in Standard, including Human Reanimator.
Dragon's Maze - AetherlingConstructed: 2
While I love a win condition with a lot of versatility and utility on it, the high cost of this might keep it from seeing too much play. You really need to wait until at least seven mana to have this protected at least a little, which is quite a lot to ask. This will probably see light play, maybe in sideboards, etc., but I would not be expecting a Titan level of play.
Theros - Gray Merchant of AsphodelConstructed: 1
This could end up being some weird win condition in a combo or control deck looking to flood the board with black mana symbols. Costing five mana is a big drawback, but the effect is unique enough that you never know.
Theros - Voyaging SatyrConstructed: 1.0
Keep on walking.