Well, literary fiction is a genre in its own right - it's doesn't just mean mainstream. Literary fiction is generaly concerned with communicating a message - over and above the quality of the story/plot, etc. In fact, in the really great literary fiction, plot almost disappears (You tried reading Virginia Woolf's 'To The Lighthouse'?). I can take literary fiction in small doses but can wallow in the escapist romps of good SF and fantasy for years...

Well there is Kurt Vonnegut, but for some reason he just doesn't feel like scifi, more like a surrealistic trip through space and time.

There is HG Wells (Time Machine, War of the Worlds) and Shelley (Frankenstein), though Shelley doesn't seem scifi either.

And arguably in the realm of fantasy Harry Potter and LOTR seems to have mainstream acceptance.