Hmmm... Interestingly, I genuinely don't know. I can find things that make me not click, but I can't pinpoint anything I especially like in a summary.
My biggest nopes are when the summary is not, in fact, a summary ("I've written this in two hours and it's 3am it's not beta read lol sorry, I hope to have another chapter soon" yeah buddy that's an author note not a summary, tell me something about your story x') ) and the "not using a capital letter at the start of a sentence ever" aesthetic (I assume that if the summary is written like that, the full fic will be too, and I just can't read something that's written that way) (I'm still confused to that day about how and why this trend exists, if you happen to know I'd enjoy having an explanation!).
So yeah, basically I want a summary to tell me something about the story, and I expect a summary with a very obvious stylistic choice to reflect the writing style of the fic (which might not always be true I suppose, but that's my reflex).
I suppose I tend to favor summaries that I find "interesting" or "intriguing" but that's not exactly something measurable so that's really not helpful x')