Over use of shiny fabrics like satin on a Inuyasha costume, Sailor Moon costume, a schoolgirl outfit(from any anime/series), Bleach costume, and things of the like.
Also I seriously dislike half-assed costumes like if you're gonna cosplay as Kefka from Final Fantasy VI. Don't show up to a convention wearing red pants and a strip of green fabric hanging around your shoulders and pretending you're him. Another, is when there are half-assed school uniforms from Suzumiya Haruhi no Yuutsu, the skirts are usually half-assed b/c no one wants to do the awkward pleats on them and people don't wear the knee high socks like how Haruhi is supposed to.
Katsucon '12 - Definitely going!
Anime Boston '12 - Possibly
Otakon '12 - Most likely
Vocaloid - Gumi (Camellia) - 100%
Fairy Tail - Wendy(Edolas) - 100%
Fairy Tail - Leo the Lion - 10%
Fairy Tail - Natsu(???) - 0%