Realistically yeah, fitting in legalities is nice. But then there are those borderlines that are really inappropriate and the what not.
Most obvious example I can come up with was "Towel Rei" from Acen 2001. She walked around in panties and had a towel draped around her shoulders covering her chest(although some people said she had a bikini top on just in case, I doubt that was the truth, seeing that I saw her multiple times and there was no way). Technically, she was covered, and many can argue that the towel covered more of her than some bikini tops do.
Would this be allowed?
Out of good taste and nature... no it shouldn't. Even if legalities DID stand on it being legal (which I'm not sure legalities would stand on), out of good taste it shouldn't be allowed. Even if it's a family oriented convention, even if it wasn't... it's just not tasteful.
Acen staff did force her to go back to her room and put on clothes.
Then we get to the broader range of taste. The idea is the same though, people that purposely whore for attention, good or bad. The Ayanami cosplayer was actually rather attractive, which was why people questioned the act. Had the girl been ugly and overweight, I doubt anyone would have said anything to her at all.
Which leads to the fun fun fun debate of Man-faye.
Man-faye was banned from Anime Expo... twice, because they felt that his costume was inappropriate for the convention. Funnily enough there were girls wearing far less clothing, and exposing themselves far more within his vicinity almost every single time staff approached him. I was present for a few of them as my ex-manager was a friend of his. One time was at the Anaheim walkway between the Hilton and the convention center. There were approximately 6 girls cosplaying the DOAX series. One girl popped out of her top and pretty much exposed everyone to her right breast. This happened no more than 5 minutes before staff approached Man-Faye and told him he had to go change. Staff was present when the girl popped out of her swimsuit, and nothing was done about it.
So the obvious reasoning... Man-faye is a hairy guy, the other was a (in this case I wouldn't say attractive) proportioned female. Not to say that Man-faye didn't do anything inappropriate, as in one incident, he lifted his leg and his testicle popped out of his shorts in the dealers hall.
So what was done right, wrong, inappropriately by staff? There is no realistic line of what is allowed and what isn't. And of course most of it will have to be just done on eye and on the spot based on taste and reasoning. But when obvious bias will be played on subjects like this. Guys will get away with far less than an attractive female. And unattractive females will get away with far less than attractive females. It sorta just goes in circles.
Really, no one likes seeing the fatties cosplaying really scandalously clad outfits, but are they allowed to? And in the same suit, what about attractive females? What draws the line between too scantily clad and just scantily clad enough?
YAY FUN FUN FUN TIMES
Appropriate for a con?
But yeah. There's hundreds of lines that can be made on what's inappropriate or not, beyond just "the costume". One can say every one of those shouldn't be allowed, and can be allowed at the same time.