Sometimes you do have to "sell" your fic in less than 100 words, particularly on Fanfiction.net. Not sure about DeviantART, though.

I really agree with you there, even though I'm usually against stereotypes, but this one has done a particularly well job of proving itself to be true.
Also, if I see tags (on Ao3) which are mini-summaries in their own right and span an entire line or are simply composed of shameless self-praise, I usually stay away.

I have a few things to add:

Barfing up keywords (especially when like seven of them are synonyms.) It does not, in fact, tell me what the story is about. It might get you a couple extra clicks, but it's extremely annoying and I've found it has a really high correlation with low-quality stories.

Begging for reviews in the summary: everyone wants their fanfic to be read. Most want it reviewed. Throwing in a cutesy "plz r&r!" just tells me that you're an inexperienced author who isn't going to take my constructive criticism if I do take the pains to go through the story.

Hedgehog's, too. If the summary asks a question like "Will X achieve Y?", you can bet that nine times out of ten the answer is yes, thus making it unnecessary to read the story to find out.

I'm not sure if there are any 100% reliable tricks to writing a good summary, though making sure everything is SPaG'd correctly is always a good start, of course. A general outline of the premise can work, but if you describe it too generally, it'll sound like all other stories with a similar plot. Quoting an interesting line or two from the story can be really effective—but if the story sucks, then your quotes will probably suck, too.

... Man, this gets me thinking, we haven't played Fill the Plothole in forever. Somebody should start a round. I'd do it, but unfortunately I don't have the time to devote to it right now. {= (

~Neshomeh

Also I must add the excessive use of questions in the summary irritates me. Like "Will Mary Sue find love? Or will she die a horrible death at the hands of the PPC?"

More of a pet peeve as well.

The Emperor Protects!