The problem is that a bad game becomes that much more difficult to ignore when it's mainstream. Halo and Half-life are crap, but if they hadn't become mainstream, other games wouldn't have so desperately tried to emulate them. As it is, they pretty much wrecked the FPS genre which I really used to like. Same thing with RPGs: Baldur's Gate -series were some of the best games I have ever played, and then along came the craptastic display known as Knights of the Old Republic, and boom, that's another genre down the drain (and, yes I know they were both made by Bioware. That only makes the pain worse, not better).
Actually I think most games I like were mainstream at the time I got into them, but the unwashed masses have since drifted away from them, and suddenly I'm an indie gamer. Which isn't to say that mainstream games of today are necessarily bad. I think the GTA series is great, and that's about as mainstream as Dan Brown. Of course the PC-versions of those games are horrible, the latest being completely broken, but hey, consoles are the mainstream now, so that's to be expected.
Still wondering why some people are just a bit frustrated with the mainstream, and just might be a bit vocal about it?