Bioshock, I thought would be "just another FPS" but it's actually really really good.

- Donkey Kong Country  

I had no interest in getting this game when it was released. I mean seriously Donkey Kong didn't have the coolest games before that and the game looked like just another 2D platformer set in some dark jungle. The SNES was full of shitty platformers and I was pretty much over em at the time. DKC blew my mind when I finally rented it. I turned it off after 2 levels and asked my mother to buy it for me right away as it was the best game I'd ever played.

- Super Mario Galaxy

I knew it would be good but I had no idea it would blow every other 3D platformer out of the water.

really SM3D Land? That was probably my 2nd biggest Nintendo let down ever after Super Mario World 2: Yoshis Island on SNES. I was so excited about it. I probably watched the demo trailer on my 3DS about 50 times before it was released. I played through all 16 worlds and can barely remember 1 level it was so forgetable.  

Batman: Arkham Asylum

I like Batman, and figured this would be another cash grab. Upon playing it I was stunned by the time and effort to stay true to source material and craft a solid game. I loved the Metroid feel of unlocking gadgets and new areas and the combat was one of the best I have experienced with multiple enemies. I expected next to nothing and ended up being one of the best games I played that year. Bravo Rocksteady.

final fantasy IX (making the best game in the series so far)
bioshock
ACII-brotherhood

Yoshi's Island was awesome and you know it!

Enslaved: Odyssey to the West. While not quite my favorite game of the generation, it was definitely up there, and I didn't expect to have nearly as good a time with it as I did. I ended up dropping almost every other game I was playing for a week or so in order to properly dive into it.

Sleeping Dogs, i just loved the setting and the story and really connected with the characters. I expected it to be good, but it was far better than i though.

God of War 2,Uncharted 2,Red Dead Redemption,MGS4,Bioshock Infinite,Shadow of the Colossus,Gran Turismo 5...

I think you're the first person to tell me that :O Yeah I honestly expected it to be a let down, I remember a lot of it!