Like you, I mostly prefer canon divergence AU:s! But I've definitely written and enjoyed AU:s with a changed setting, too. I like it when you introduce some new worldbuilding element that can interact in interesting ways with the characters. For example, I remember in due South, someone said my AU where Benton Fraser was a werewolf made more sense than canon, which was quite a compliment! : )

I don't like changed-setting AU:s which undermine or remove the things about the characters which make them themselves or which make their interactions too different, if that makes sense. For example, if you take characters from a war setting and put them in a coffee shop, that is not likely to work for me.