There were 2 games I hated the demos on but Batman AA and inFamous turned out to be fun games. I am also another person who didn't think I'd like Mass Effect 2 (and the series), but I ended up loving it! Then again, I also thought I'd really enjoy Enslaved, but it just sort of fell flat for me and I had to motivate myself to finish it. Same with LA Noire, a big meh....
I, for one, welcome our new feral ghoul overlords! (call me PD -it's easier)
Sly Cooper, thought it was a kids game when I first saw it, didn't realize it was a really good platformer till I saw a buddy playing the hd collection about a year ago, I've since beat the first 2 and aim to beat the third before the fourth comes out.
Every screenshot and every video I saw, as well as the soldier on the cover, presented the game as if it were just another generic military FPS, with maybe a few RPG elements thrown in. Hell, this very site lists it as " Role-Playing » First-Person". And to me, that was just a huge turn-off. I'm just not the first person type.
So naturally I play it, and absolutely love it. You're able to play in 3rd person, and it feels just as much, if not more RPG than Shooter. Hell, you don't even have to use guns. Great soundtrack too.
John Lennon, 1940 - (Infinite Symbol) PSN: naathaann
Recently? DmC and Ni No Kuni. Hated the demo for both of them. Didn't like the idea of DmC in general. Heard NNK was like a mixture of Pokemon, FFVII and Dragon Quest; three of my least favorite games. Turns out I love them both.
Can I use an entire genre as an example? Lol. I've always hated racing games even though I'm usually pretty good at them. Just never enjoyed them for some reason, even as a kid. Picked up Burnout Paradise when it released and it changed my entire perception of the genre. Been playing it off and on since. Recently picked up NFS:MW 2012 and it's made me love racers even more.
Besides that, there have been hundreds upon hundreds of games over the years that I've been totally wrong about (both positive and negative).
Ni No Kuni, Vanquish, DmC, Sound Shapes, Disgaea 4, Valkyria Chronicles, Need For Speed: MW 2012, Far Cry 3, Monster Hunter FU PSN: DocDelicious