i have been thinking about AUs recently (my very-long-plotted, still-utterly-unfinished ones, heh... *headdesk*), so i'm very glad you made this post! :D i agree with pretty much everything said above about certain types of canon lending themselves well to AU, though i can't really pick out the characteristics that make them work (is it flexibility? cracktasticness? what??). but yes, either way: grasp of characters and premises vs. canon makes quite a tricky combination, so i have metric fucktons of respect for the people who write AUs and write them well!