I want a summary, that, well, summarizes the story. Shorter fics can get away with a snippet of dialogue/narration or something more esoteric (like a dictionary definition of a relevant word). But with longer stories, I want summaries like you'd find on the back of a novel. That's what I do with my own works, at least; since I don't write pairings/fandoms that draw a large crowd, anyone from outside the fandom I can draw in with an intriguing summary is great for me.

Summaries alone won't make me skip a work (unless the story just doesn't hit that 'gotta read' urge), but a lackluster summary won't convince me to read one, if that makes sense.