For the mostpart, i hate mainstream because 'mainstream' players don't want the same things from a game I do. COD4 continues to be the best example here, people want big explosions, and good graphics combined together to create constant chaos, personally that just pisses me off, ok war can, and is chaotic, but at Call of Duty rate the world would wipe itself out in a couple of weeks.
Same for films, 'mainstream' tends to me good special effects, with good looking actors. Whilst i would rather have a good storyline/characters/intresting camera work/direction, pretty much anything other than just special effects. The best recent example of this would be Wanted, which i still haven't seen, so can't judge too much, but it really does look awful.
This of course doesn't explain why i'd hate mainstream, only why i wouldn't like it. The reason i hate it is that everyone else won't shut up about things that i can see no value in, which obviously gets irritating, and makes me hate it.