See, things like Twlight and Bieber, I hate because I hate. I could give you any number of reasons - Twilight is poorly written, it's self-insert author fantasy fulfilment, what's-her-face can't act for peanuts, and so on. Bieber's lyrics are repetitive and shallow, the melodies are processed and unoriginal, the rhythm is flat, et cetera et cetera. I don't hate them because they're mainstream.
In fact, I like plenty of mainstream things. I'm a Glee fan, for one. I don't like or dislike things because they're mainstream. I dislike much of the mainstream because of the things it contains themselves. For example, the aforementioned Twilight and Justin Bieber, as well as the rash of crappy vampire romance fiction and singing teenage boys that were born simply as an opportunistic method to buy into society's current desires. That is the very definition of mainstream.
In fact, I give Stephenie Meyer more credit than the authors of this plethora of vampire romance novels we're seeing in the Young Adult section these days. While what she wrote was undoubtedly crap - from my point of view, at least - she was writing it because she wanted to write it. These other authors are just jumping on the bandwagon. That, if anything, is the reason I dislike the mainstream. It's not people writing, singing, drawing what they want to write, sing or draw. It's people writing, singing and drawing what we want them to write, sing or draw; or worse still, what they want us to want them to write, sing and draw.
Indie music, for example, is an almost bizarrely rich and diverse genre, simply because, for the most part, the producers of said music are making the music that they want to make. On the whole, it is simply more real than most so-called 'mainstream' music. That, for me, is the definition of mainstream. Not where you sit on the charts, but how you got there and what it means to you.
The majority of mass-produced media in the world today is designed to appeal to the lowest common denominator - that is, it aims to please everybody by lowering its standards. This, however, often sacrifices quality in the eyes of the discerning fan. Much mainstream music, for example, is formulaic, repetitive hip-hop, indistinguishable from any other song by seventy per cent of the well-known artists in the world today.
Well-known, however, does not necessarily equate to being crap. I look at this with an open mind - I enjoy several 'mainstream' bands such as Linkin Park, Paramore and Hoobastank as well as lesser-known groups like supercell and The Wombats.
I like bands, films and novels because they're good, not because the media tells me they're good. If the media tells me something is good, and I decide that it is, then that's fine. If I hear about it some other way, and it turns out I like it, then that's fine too. I generally try not to distinguish between mainstream and non-mainstream. It just brings up drama that I'd rather not be brought.