I am almost never interested in mundane AUs of SFF canons. To me they're like, "Take this thing I enjoy, but make it more boring." I read SFF for a reason - I like the genre a lot. The only exception would be if the AU setting was something unusually interesting to me AND if it was really well-done. There's a great film noir AU of Saiyuki for instance. But it's usually not my thing.

On the other hand, canon-divergence is one of my favorite genres. And I often like "a different type of SFF setting" AUS, like fantasy instead of SF or vice versa. Part of the fun of those is seeing how non-realistic elements get translated between settings, like a dragon becoming a sentient spaceship or vice versa.