I like summaries that describe the premise but nothing more, so I have the general idea of what the story is about without getting spoiled. I look up the summaries for movies and tv shows, the kind that pop up on a tv guide or the blurb on a google search, and try to emulate them for my fics when possible. They're usually two or three succinct sentences that describe the premise and use language that fit the mood of the movie/tv show.

For example, Meet the Parents, just two sentence with a bunch packed in: A young man's first visit to his girlfriend's parents' house turns out to be more nightmarish than he could ever have imagined. Hoping to use the weekend as a chance to propose to his girlfriend, he only succeeds in incurring the wrath of his prospective father-in-law and almost destroys their home in the process.

The only summaries that irritate me enough to not read a fic is for a 50k+ fic with a summary like Merlin evades, Arthur suspects, and at this rate, they'll never finish the quest. I can accept that for a fic that is less than 1000 words or else the summary might wind up longer than the fic. lol