Disney Epic Mickey.

I was a slave to reviews back then and avoided it on release due to its critical thrashing.
When I borrowed in a year later, I was taken aback at how awesome it was. That was the moment I stopped basing my purchases on review scores.

Uncharted - For me, it felit likes Nintendo EAD making a realistic game. I doubted Naughty Dog was gonna have success with those game. Boy, was I wrong.

Red Dead Redemption - I never thought GTA in the Wild West could be this fun. Through the Manimals were really crewpy

Gears Of War - I always thought third person shooters were bad because of their cameras angles, but this changed my mind.

Monster Hunter Portable: I didn't think that a gem with no stor could be this fun! I'm a big fan of ganes that have a story, so I thought I would sell it immediately. I was so wrong.

Kingdom Hearts: 368/2 Days: The only Kingdom Heaets game I played on a handheld was horrible, so I thought this would be to. It suprised me in their fact that it needed no analog stick or touch screen to move in a 3D world. Great story too.

Fallout 3 was the only real big surprise where i had mediocre expectations and it turned out to be in my top 3 list for the gen.

Red Dead Redemption. i came in knowing nothing about the game. boy was i surprised.
Killzone 2. we got the game with the PS3, boy was i surprised.

Nier. I think it's one of the most underrated game of the past 2 generations.

I thought it'll be alright, and it turned out to be really great.

I expected nothing from these games, but they really surprised me:

Dragon Age: Origins - I used to think PC RPGs (Or WRPGs as most people call them) were garbage. I saw my brother playing it, and decided to get a copy for myself. Easily one of my best gaming purchases of the 7th gen.

Flower - I never knew something so simple could be so amazing.

Batman: Arkham Asylum - I still don't like the roided up character models, but this was an AMAZING game. I loved every second of it.

Infamous: Second Son - I hated the first one and I thought the second was decent. But this one blew me away with fun factor.

The Last of Us: Remastered - I've never bought a survival horror game until this one. The genre just never appealed to me. Maybe it's because this game is more survival than horror, but it hooked me nonetheless. I still think about it even though I finished it a while ago.

Sleeping Dogs - Still the best sand box game since San Andreas.

Yakuza 3 - I never played the first two, but I was enthralled with this masterpiece. Where the fuck is Yakuza 5 and Ishin SEGA!?

Armored Core: 4 and 4Answer - I always said that the Armored Core series was missing something important. Speed. And these two games provided it in quantity. Some people didn't like that, but I love it. It's a shame they went the other route with 5.

Vanquish - High Octane awesome. Enough said.

Binary Domain - I. Need. A. Sequel.

Dragon's Dogma - Shamefully underrated. The game is just so special, I can't really describe it.

Kingdom's of Amalur - I really hope they make a sequel, because I rarely put over 80 hours into games just doing side quests and exploring. this is one of the few games on that list.

COD 4 - I honestly never heard of the series before I walked into GameStop and saw all these people lining up to get a copy. I decided to get it on a whim and I'm so glad I did. It was just SO much fun. I hated first person shooters before this game and I kind of still hate them, but I found some gems because of this game.

Killzone 2 - This is the first FPS multi-player that actually hooked me in. I used to play it for hours and hours every day, because it felt like I was fighting in a mini war. No game has come close to mimicking that feeling for me yet.

Assassin's Creed 2 - The first one was repetitive garbage, and I didn't really know what to expect from this one. What I got was one of the most engaging games I've ever played.

Uncharted 2 - I watched my brother play through the entire first game (we only had 1 tv in our apartment at the time). It looked cool, but it didn't really grab me. This one though. Holy shit was it awesome. I rarely play through singleplayer games more than once, or on higher difficulties than normal. This is one of the few games that made me do both.

Games that surprised me in a good way?  Nice topic.  I will try and leave out the big hitters that everyone knew were going to be good anyway.

Art Style: ORBIENT - This is such a simple game, but it was completely addicting.  All you do is play around with gravity.  You play as a star, and you only have two control options, 'A' for attract and 'B' for repel.  You absorb smaller stars to grow in size or catch them in your orbit to score points, all while avoiding crashing into larger objects or getting sucked in by black holes.

de Blob - One of the best 3D platformers I have played not named Mario.  Who knew painting could be so fun?  Great use of visuals and music.  It is very rewarding to turn the completely white environments into a rainbow of colors.

Fluidity - Fun little puzzle platformer.  You play as a puddle of water.  Yes, a puddle of water.  You use tilt controls to move the water around.  Eventually you get new powers, like turning into a block of ice or becoming a cloud of steam.

LostWinds: Winter of the Melodias - This one is an exploration based platformer.  It makes great use of the Wii remote IR pointer to control a wind spirit that helps the little boy you control with the analog stick.  The first was good, but the sequel felt like a big improvement.

Excite Truck - It was just a launch title, but it was a heck of a lot of fun.  Just a crazy racing game.

Muramasa: The Demon Blade - I wasn't too sure what to expect of this one.  I heard it was good, so I gave it a try.  It turns out it is just a fantastically well made, high-quality game.  Beautiful visuals.  Amazing boss battles.  Tons of fun.

Oh damn, I forgot to mention one of my favorite games of 7th gen: Half-Minute Hero, on the PSP.

Holy shit, what an incredible game. The plot, despite being really simple, manages to be entertaining and humorous (and it even has an "epic" feeling to it). Gameplay is ridicuously addictive as well.

Spec Ops The Lines was amazing. On a technical level, not the greatest games (visually and gameplay wise) but the story, pacing and settings were fantastic. It questions your morality on every mission and leaves you feeling hollow by the end.

Dissidia Final Fantasy was just tons of fun. I spent a combined 300 hours or so on both games for the PSP.