I think the only thing I would maybe add is that the difference (for me) between a good AU and one that I back-button out of is whether the author knows the difference between a premise and a plot. There's plenty of AUs that would be neat (what if they were all magical girls?) if only there were some plot to go with the shiny premise (...and they had to deal with $dilemma?).
A premise can support a scene or two; a plot can support a full-bore fic.
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I think the only thing I would maybe add is that the difference (for me) between a good AU and one that I back-button out of is whether the author knows the difference between a premise and a plot. There's plenty of AUs that would be neat (what if they were all magical girls?) if only there were some plot to go with the shiny premise (...and they had to deal with $dilemma?).
A premise can support a scene or two; a plot can support a full-bore fic.