A few loose pennies of thought on this topic.
ONE: Law enforcement, including FBI, may be reading this forum. They may be all over 4chan, too. The unfounded belief that "it's only an internet meme, and only the cool kidz who know it's a joke would recognize it" explains why morons who do stupid vandalism, film it with their camera phones, and post clips on their Myspace, then suddenly get surprised when folks with wallet badges visit them (or their parents) with uncomfortable questions or duly deserved charges to press. Kumoricon really doesn't need this sort of baggage.
TWO, relating to ONE: Back in the 80's a bunch of college students invented a game system called 'T.A.G' which stood for 'The Assassination Game.' A list of players would submit their class schedules to a game master who would pair off students for 'hits.' Using water pistols or other creative devices (an envelope with a taut elastic and a button, so that opening the envelope spun the button and made a buzzing sound - was a 'letter bomb,') to rub out players. The game master pared the players down and announced the final survivor. Cool fun, then players started painting their water pistols flat-black. NOT VERY COOL AT ALL when a player gets into the role, shouts "YOU'RE DEAD, SUCKER," at a campus bus stop, draws the realistic-looking water pistol ...
... and gets dropped by REAL bullets by an off-duty cop whose training takes over. This actually happened, and the popularity of 'T.A.G.' absolutely and instantly cratered. (Oh wait, they made a movie called 'Nikita.')
Now what's happening is that people who want to play with prop weapons (honest, innocent, above-board fun) color their props. What then happens is that creepy bent dirtbags spray paint their REAL, toy-looking Glocks in these same silly colors, so they can sneak them into a situation where everyone expects colorful non-functioning toys, and caps people FOR REAL. I'm just waiting for some nut case to bring in a real gun (stolen) to a paintball alley and start whacking out innocent players.
Morale of the story is that when law enforcement skims a forum board or a stream of IM chatter, and they read what *might* be a *credible threat,* they really HAVE to prepare and react. "It's only a joke" gets about as funny as time-bomb 'jokes' would be at the airport. Technically it should be free speech yes, but you gotta admit that that would be quite the bonehead manouver, and you couldn't really be 100% surprised if anyone who has to act as a community custodian (meaning law enforcement, hotel staff, etc) takes a 'joke' seriously. And then the party's really over, because people responsible cannot tell the difference between a joke, versus a very clever person trying to do something *for real* under the best possible cover: "It's only a joke!"
THREE: Just my humble opinion on the Nazi costumes. I didn't see any, but I did see a Jin-Roh character. I haven't seen Jin-Roh, so I would not attribute their Nazi-looking costumes with the evil goals of the actual Nazi party: racial stratification, eugenics, genocide, phrenology, and their weird distortion of Nordic legends to their own fanatic ends. Jin-Roh does equip their infantry with the German MG42 medium machinegun, 8mm x 1200 rd/min fire rate, which as a historical collectible is rather cool. Also why the few real ones available in this day are
EXPENSIVE.If I saw a Nazi costumer at an anime con with no other context, I'd reach and possibly remember URDA (Don't bother watching it - another just-my-opinion) but I'd also wonder whether that person was clueless enough to no know he might make people uneasy - especially people alive today who had uncles and grandparents who were gassed to death. Or shot.
Next, I'd think that the Nazi, when in the company of either a set of other people costumed as known baddies: Hitler among Fidel Castro, Josef Stalin, Pol Pot, etc. or a Waffen SS trooper in the company of peers and adversaries, such as a few Allied soldier cosplayers, Italians (Axis,) Austrians, etc. then the point would be to 'complete the set,' rather than to call attention to one (particularly objectionable) worldview, and in the context of a set, I'd find it less unsettling.
Oh well.