This is an interesting question. I've thought about it quite a lot over the last three weeks.
Disappointments are fairly rare. Thanks to this forum, a couple gaming podcasts I listen to, and GiantBomb's Quick Looks, I'm pretty keyed into which games I'm going to enjoy. It's not like it was before the internet, when I had to rely on the box shots to see what a game was like.

Still, once in a while, something slips the net.
Disappointments-
Bayonetta -
This one had a good demo, good Quick Look, good buzz...and just wasn't for me. Lots of sound and fury, but very little substance. Maybe I just never got a total grasp on the combat system, but it was never really clear to me why I would sometimes get slaughtered, and other times completely decimate enemies without taking a hit. I never knew if Witch Time would trigger when I wanted, or why it sometimes lasted longer than others, or why the attacks that worked so well before were totally ineffective on the replay.
The story grew wearisome, as well. Where
Ninja Gaiden offered just enough exposition to give you an excuse to hack bunches of dudes apart,
Bayonetta's cutscenes can be five minutes or more of nonsense. All the frenzied action and incomprehensible blather was cute at first, but as the game wore on, the craziness felt like a substitute for quality. If you can't think of anything good, just put up the silliest things you can imagine.
It sounds pretty dire.
Bayonetta isn't a bad game, though; just disappointingly mediocre.
-
The Force Unleashed II -
FU2 is an amazingly good-looking game. Everything about the lighting, textures, and animation looks phenomenal. It plays better than the first game, too: the saber swings are still a bit too floaty for my taste and there were a few moments when I had trouble throwing something towards the right target, but it feels much faster, more responsive, and more in line with the source material than the original.
The disappointment here is that there's not nearly enough of it. I'm a pretty slow gamer and I finished the campaign during an otherwise busy weekend. Playing on Hard. Subtracting the occasional death, I'm not sure the whole game is much longer than four hours. I don't think it was a lack of ambition, either: throughout the story, there are signs that large swaths of the planned campaign were torn out and the gaps hastily taped together with recycled content and some throwaway line of dialogue. We're left with one-third of a full game, and given how well that part is done, it's pretty disappointing they lacked the time, funds, or talent to see it all the way through.
-
Elemental: War of Magic -
I was hoping for a cross between the wizardry-themed empire building of
Master of Magic and the challenging AI and customization options of
Galactic Civilizations II: Twilight of the Arnor. What I got was a ponderous, dreary mess that is only now, months after release, being patched into a playable state. Good on Stardock for pushing ahead on their commitment rather than cutting their losses, but it's still one of the biggest gaming disappointments of 2010.
- Gratch Didn't Like
Persona 3 -
Man, I really thought Gratch was going to like
Persona 3! He's been in such an RPG funk, I thought
P3's quirky characters, monster-fusing mechanics, and touching storylines would win him over like they did me. They didn't, and while that doesn't really affect me, I still wanted him to have a good time and I'm disappointed he didn't.
Surprises-
Just Cause 2 -
I really hated the first
Just Cause. I mean *hate!* I can't play it for more than fifteen minutes before its ugly graphics, awful controls, and tedious navigation drive piss me off beyond reason. I thing I bought it from Circuit City for $5.00 with the intention of farming it for a few Achivements, and I can't even get that much out of it.
Just Cause 2 is freaking amazing. An entire country with geographically distinct regions, packed with settlements to liberate, supplies to find, races to beat, and story missions to complete. It's hard to think of another game that combines aerial, vehicular, and run-and-gun gameplay so seamlessly, and it's one of the most beautiful games I played this year to boot. I spent over 81-hours earning a
97% Completion rating in
JC2, and just last week, I bought it on sale for the PC so I can start again.

-
Assassin's Creed: Brotherhood -
I couldn't have cared less about this game during its development. I was sure it was a quick cash-in to eke a few more dollars out of the
AC2 assets while they worked on the real sequel. And even after the early buzz started to build, after I started hearing about how big the story mode was going to be, I was absolutely *sure* I didn't give a damn about the multiplayer.
AC:B was only the third game in which I earned every Achievement. It's the only multiplayer game I've ever played enough to hit the level cap. I earned 100% completion in every story mission and side quest, collected every flag and every treasure, and earned gold medals in all the bonus challenges. I'm really amazed that a game I had such low expectations for could have turned out this well, to the point where it's one of the best games I played all year.
-
Persona 3 -
I'd always thought I would like
P3, but after failed attempts to get into the PSP remake of the first
Persona and
SMT: Strange Journey on the DS, I started to have doubts. Slogging through cryptic mazes with random monster battles every eight steps is incredibly tedious RPG design, especially when the story is so slender, the only reward for toughing it out is the opportunity to advance into being killed by harder enemies.
Persona 3 is an incredible game. Even at the height of my expectations, I would have been surprised by how thoroughly I fell in love with it. I'd wake up early in the morning to push through my workout so I could fit in a few battles before work, and set up the DVR so we wouldn't be hurried through the next Full Moon encounter that evening. I already
wrote all about it here, and six months later, I think
Persona 3 is my favorite game of 2010.
-Autistic Angel