I Suck At Summaries
If you can't write a decent summary, how the hell am I supposed to expect you to write a decent fanfic? Geez, at least if there's no summary at all, that way I can assume you're just lazy.
Most works described in this fashion, needless to say, aren't very good. Furthermore, such stories usually die long before they are completed, due to the author giving up from lack of feedback. Neither is surprising: if the author lacks the skill and dedication to write a proper summary, why should the story itself fare any better?
Sometimes followed with "please read and review", although most fanfic readers take the phrase as an ominous warning and end up not even reading the fic, let alone giving any feedback. Sometimes, the phrase is substituted with "Summary sucks, story much better" or something similar, but most readers will react in the same way as before.
This should be avoided since it tells readers that either the author doesn't have any self-confidence, or that the story is so boring that the author couldn't be bothered to put in a real summary, or worse, that the author just sucks. It also shows a failure of Show, Don't Tell: don't use the summary to say the summary is rubbish -- write a better one that summarises the story.