I know this is an old thread but I want to resurrect it. It's a good discussion.

The first thing that makes or breaks a book for me is the language. Some authors have the ability to make me read slowly and savour every word. A professional author who really drags me in this way is Edith Pargeter, creator of Brother Cadfael (under the pseudonym Ellis Peters). A fanfic writer who does it is 'Alicia', whose Sherlock Holmes slashfic is archived at No Less Remarkable.

The second thing is that the characters have to behave in ways that make me like them, understand them, or just generally be intrigued by them. If they're too predictable (note that 'in character', when it's fanfic, does not count as predictability) or if they do inexplicable and stupid things to make the plot go the way the author wants it to, I'll lose interest.

As long as those conditions are met, I can be entertained by a wide variety of wacky plots and premises in pretty much any genre. In general, I think all of this points to my wanting to be shown that the author is thinking things out. If someone is out of character, there has to be a reason for it; or if the premise has some weird or fake science in it, I have to be able to believe that things do work that way in that universe.