I'll put the two extremes on here.

What I thought I'd like but loath:
Final Fantasy XIII series

Thought I would be indifferent about but turned out to be my favorite game 7th gen no contest:
Xenoblade Chronicles

When it comes to games I thought I'd enjoy but I ended up disliking Golden Sun comes to mind, the game looked like an all around fun traditional RPG with neat visuals for the GBA, I ended up never finishing it and having very inconsistent amounts of entertainment out of it. Mario 3D land is another example, I thought it'd be a fun 3D Mario, it ended up being as generic as a game can get and probably my least favorite game in the entire series, with Sunshine being the only other game in the series I find as unenjoyable. I guess you could add Final Fantasy IV and Chrono Cross to this list too.
I'd say Dragon's Crown is a game I expected to dislike due to how flawed Vanillaware games usually are, but I ended up really liking it, being one of my favorite PS3 games and games of last year.

I thought I'd like Other M.
Never have I been so wrong.

The two biggest surprises for me actually came from the same series, Killzone.

I rented the first Killzone in order to play Killzone 2, but wasn't very optimistic about it. Surprisingly, I fell in love with it. I assumed that I would love Killzone 2 even more, considering the first game received mixed reviews and the second one was a critical darling. But, unfortunately for me, the sequel was a huge disappointment.

I thought I'd love Bioshock, GTA IV, and Red Dead Redemption but none of them were good.

I thought I'd love Super Mario Galaxy 2. It was praised far and wide as not only the perfection of the previous game's mechanics, but as a perfection of 3D platforming as a whole. Game of the generation, blah blah blah. I didn't necessarily hate the game itself immediately upon playing it, but I didn't have all that much fun playing it, certainly no where near as much as the first game, and left extremely disappointed, and consider it the least fun I've ever had with a 3D Mario platformer. This and the fact that Nintendo choose to make a second, watered down version of Galaxy 1 instead of a brand new 3D Mario game makes me really hate it. I know it's an unpopular opinion, but I stand by it.

I didn't really expect to like Tales of Phantasia, but that was before I really knew what RPGs were. It quickly became my favorite game and got me addicted to the genre. I tend not to think I'm going to hate a game I play because why would I get it then? ToP was an exception because it was when I was little, and my mom got it for me because she saw me eyeing the pretty box art. Don't always count out a game your parents get you on a whim, kids, it might just introduce you to the love of your gaming life. Don't knock something until you try it or at least have a good idea of what it's like. The thought that I even doubted that game for a second breaks my heart.

Thought I'd Hate: Twilight Princess, Uncharted series, Ratchet & Clank: A Crack In Time
Thought I'd Love: Sonic Lost World (But It's still pretty good, just dissapointing)