A lot of the really good ones are by proffesionals and by models that, in some cases, even get PAID for cosplaying. Point being, of course some of them are going to be exceptional if its there job. Not to say there aren't some amazing ones anyways :D
I DJ at several anime cons and I know a few of those girls. Man, the Youruchi Shihouin costume is hot, so's the Amane Misa one with the Death Note...omg..I'd do her so fast...