Probably a big turn off is having to sift through a lot of badfic to find the gems. That's why I like some of the sites with a quality rating system that will reject the really bad ones. Most of my experiences with those have been good (particularly one site that was dedicated to Final Fantasy VIII fanfics that had nice quality control).

I've written a few fics myself. Nothing major or hugely multi-chaptered or anything, but I always tried to avoid all the pitfalls of the badfic. I'm a little sad because I got turned down from one such controlled website, but that was for an Evangelion fanfic I wrote and they denied me only on the basis that the idea I'd written the fic about had been debunked in the canon. Which.. seems to not make sense, since it's fanfic! XD Ohwell!