Intelligent, independent, competent heroines, especially if they have a life other than being the hero's lover. I want to feel he's the best choice for her, not that he's her only choice.
Characters with a sense of humor.
Heroes who are different from the norm in some way.
Scientist main characters (characters who use science to save the day, rather than, say, assassins pretending to be physicists).
Complex plots where I can't be sure how the problems will be resolved.
Realistic, detailed settings and worldbuilding.