I like attending costume parties. Unfortunately, a lot of people simply have no creativity when it comes to costumes.
Consider this pre-Halloween dance that I attended recently. There was a costume contest at this dance, and three of the women who joined the contest were dressed up as witches. Get that? Witches. Plain old boring witches in black outfits. There is simply no way that a costume like that would win in any sort of honest contest.
Now if they had dressed as the witches of Eastwick, that'd be different. Or maybe Wendy the Good Little Witch. But your garden variety witches? Boring as all get out.
Costumes that are too obviously store bought are also extremely lame. Batman costumes, for example, are almost invariably lame, especially when they involve fake muscles. (Here's a hint, guys. Fake muscles on a costume are always a dumb idea.)
I know a guy who dressed up as Robin -- the classic Dick Grayson version. That's more original than dressing up as Batman. What's more, the costume looked very authentic, since it was homemade and had a lot of effort put into it. (Store-bought Bat-costumes invariably look cheesy.)