My first fandom was Final Fantasy. In the FF worlds magic and monsters are commonplace and I have a lot of fun writing this kind of fic; however in other fandoms this kind of stuff has to have AU labelled on it.
Some people seem to flee from AU without giving it a chance. Personally I'm willing to read AU as long as the world is well thought out and there aren't any blatant flaws in the logic.
What are your opinions on AU? Do you read it or avoid it? Do you think AU fic is inferior to normal fic in any way? What makes a good/bad AU fic in your opinion?
EDIT:
Since AU is a little vague I personally break Alternate Universe down into three sub-sections:
Copy and Paste!AU - Taking characters or themes from one fandom and putting them in another fandom. E.g. Taking daemons from the His Dark Materials world and applying them to your favourite TV show, or putting Spock and Kirk in Middle Earth.
What if?AU - Changing an event in canon and following the story to see what changes will occur through the ripple effect. E.g. What if Tara hadn't been shot by Warren?
AU!AU - Changing the universe around the characters, keeping them the same or only making changes to make them fit the universe. E.g. The characters of Firefly in the old West, River would lose her psionic enhancements but could still be characterised as crazy.
Do you classify AUs in any other ways?