Good villains. If I read an original story it has to have plot and good antagonists. (My story doesn't particularly have good guys and bad guys itself, just seven goals that often oppose allthough periodically respect or aid each other.)
That's all I need, because if someone's adversary is formidable there's a good chance she's formidable as well.
In fanfics? Ingenuity. I hate reading cookie cutter plots or standard pairings, I want to see a skilled writer do something old and make it new or take on something normally ignored or even create their own new little niche.