Personally, I don't hate anything because it's mainstream. I do however hate how people treat it like it's the best game ever because it's mainstream. To fit in with the cool crowd, if you will. I was even guilty of this when I was about 13 or so.
Point being, mainstream games aren't bad. They're of good quality for the most part, not broken, and at least original in some way. It just depends if it's your cup of tea or not. For instance, I think GTA IV is to samey, Halo is lacking in good singleplayer, and Gears is cheesy as hell. However, I know people who like that kind of stuff. That's why I loved Bioshock. I'm a sucker for great setting and story. So, liking certain mainstream games doesn't mean that you're an idiot or anything, it's just what you prefer. Holding the game like it came from the hands of God himself, well, that's a different story.