Professionalism is a big part of it. If it's clear that the writer didn't put much thought or effort into the work...I don't care for it.
What else do I look for? Well, it must be a compelling story (or scenario, for a short "vignette"), with characters that
ring true. Established characters must be recignizable as those characters, and original characters must be handled with similar integrity. Compelling, authentic--that's the thing.
And dialogue. I must admit--I'm big on dialogue quality. If it clinks, I turn off. If it's great, I can forgive a lot. (Helps explain why I'm so forgiving of
X-Men Origins: Wolverine, but despise
Star Wars III: Revenge of the Sith like you wouldn't believe.
Dialogue.)