Ooh interesting! I don't think I've ever read fic without knowing at least a little bit of canon.
I agree though about crossovers because I have read those only knowing one fandom. They are a great gateway drug into other fandoms.

Soo I think a good example of a canon divergent AU would be lanning's Identical series. You should know the basic relations between the characters (at least Clark and Lex) but the rest is wonderfully explained or introduced, plus there are wonderful OCs anyway. And the writing is SUPERB - plot, subplots, characterisation - anyone could enjoy it for that alone I think.