I dont really know much about the whats mainstream now a days. I would have to agree with chaosyoshimage, in that its called mainstream for the obvious reason that it is main or very popular. Personally, I stopped listening to the radio years ago and only listen to what I knew previously, what I discover, or what is recommended to me by friends. For example, I just found out about the singers Rhianna, Drake, Kesha and a variety of others about 3 months ago during a music class in college. Before that class I had no idea who those people were.
I honestly dont care about genres, if I like the song I will listen to the artist.
I listen to Animals As Leaders, Gojira, Tyr, Chopin, Brahms, Debussy, Al Green, Daft Punk, Yoko Kanno, Turnabout Jazz Soul, and the OST of different anime's. (Gungrave, Trigun, Cowboy Bebop)