Games you didn't expect to be good, but ended up surprising you.

Costume Quest. Just put it up to try it out quickly like most indie games, and bam, loved it.

Let's see...

And Yet It Moves
Bastion
Deponia (Complete Journey)
Civilization V
GRID 2
Mafia 2
The Treasures of Montezuma 4
Trine 2
Dust: An Elysian Tail
Mark of the Ninja
Blackwell Deception
F.E.A.R. Franchise
Strike Suit Zero
LIMBO
Jolly Rover
Papo & Yo

I think that's it. I think.

Halo

Little Inferno (don't why but I felt happy whenever I burnt something)

Tiny & Big:Grandpa's leftovers (thought it would be shitty. I was wrong though)

The Culling of the Cows (Slaughtering cows is fun)

Batman Arkham Asylum (I thought superhero games are shit but I was proved wrong after I played this game. It's a masterpiece. This is the kind of game that every superhero deserves.)

Tales from Borderlands

Arkham Asylum

La-Mulana: in fact, when I first played the original freeware release, I was unable to get into it at all. I gave it a second chance and this time it really grew on me, so much that I paid full price for the remake when it appeared on GOG.
I also didn't have much hope for Saints Row: The Third. I considered it some silly garbage that came in a bundle I bought for something else, but turned out to be pretty fun when I actually tried it.

Defy Gravity: I got this one on sale because at the time I could sell the trading cards for a profit. I ended up reccomending it to a friend because it's one of the best platformers I've ever played.

skyrim. the next is beyond two souls. hopefully that game suprises too

Mount & Blade: Warband
Even though it had lots of positive reviews, I never expected it to be that good. Now it's my favorite game!

That kind of game was for me.... Little Inferno :)

Battleblock Theater
Super Panda Adventures
Papers, Please

There might be some other ones but those are the only titles that come on top of my head at this moment. Besides, I still have (too) many games unplayed in my library so I guess there are other little gems waiting to be picked up. ^^

Pathologic

Journey of a Roach. :P

Machinarium, i love it
Botanicula, i tough ill love it for one night, but ended up becoming my wife.

Asura's Wrath

Audiosurf

Im very careful with the games i put in my library, got these from the review hype and i agree with you people:
Brothers a tale of two sons
Deus Ex Human Revolution

Some others:
Tropico 4
The void (mesmerizing, love the voice actor).
World of Warcraft back in time.

FEZ - My hatred for Phil Fish immediately put me off from playing the game but when it came around as a Playfire reward I tried it out. It was pretty good.

Eufloria

Skyrim is the one that comes to mind. I hated Morrowind. Hated the gameplay. Hated how it had no direction. Yet I reluctantly gave Skyrim a try and ended up loving it.

Thomas was alone.

Sleeping Dogs was a lot more enjoyable than I imagined, I've never been very keen on that sort of combo fighting style but I can't deny the game did it well enough to make even me appreciate it!