I don't buy that at all. These are professionals, doing a job. Lots of people work for long periods of time with other professionals without it turning into The Dating Game. In any event, as per the OP's question, I'm not trying to read "romance in space." It's rarely done well, and it isn't what I'm looking for when I read Trek.
They're Starfleet officers, not monks under a vow of chastity.
Anyways, on the topic of the OP, I look for the same things when I read ST fanfic that I would in any other fanfic or fic in general: quality writing in the technical sense (spelling, grammar, etc), a reader-friendly writing style (trying to read and enjoy a story should not "hurt"), strong grasp of character, plot and background, etc.
Fanfic should not be judged any differently than any other fic.