I think I agree with what almost everyone is saying. Mostly, I think it comes down to putting a little bit of personal flair into a costume. I've done costumes completely homemade, handsewn, designed & everything, but I've also gotten store bought costumes, and then made my own adjustments to them to make them unique.
I suppose if you bring the right attitude into any costume, it's bound to be great!
ps....I'm thinking of doing a female Robin this year.....I'm sure I'll have no trouble finding a Batman out there!