I like fics that stick with canon characters, but in unique storylines (as opposed to the plots and plot ideas that tend to get overused in fanfiction). Also slice-of-life silliness and crackfics. Moczo wrote several of my favorites, including Happy Riniversary and Sick Time.
I take it for granted that writing will flow well and sound ordinary, unless purple prose is used intentionally for comedy - see Thirty H's. It's a quality that's hard to describe, and I only really notice its absence; I never come away thinking, "That was some good writing! It was NOT awkwardly phrased!"