I've never thought to warn for slash. I figure giving the pairing and rating makes it pretty clear that you may or may not want to read it. The main thing I warn for is for lack of Spuffyness, since that's my main focus and if I'm going to throw a Spander or Spack in there, I figure I'd better let my regular readers know that it's not going to be Spuffy. I find myself reading more and more gen fic now - the story being more important to me than the pairing(s) involved. A good writer can probably make me believe any (reasonable) pairing throughout the course of a good story - and if he/she can't make me believe it, then I probably won't finish reading it anyway. I don't seek out other pairings, but I'll read them if I trust the author.