io9 thinks they’ve found it:  uniforms.

Or more to the point, some kind of look that is instantly recognizable and fun.  It could be the primary color uniforms of Star Trek or the uniforms from Battlestar Galactica or Babylon 5 (although I’ve yet to see a bunch of those in convention pics…strange.)  Klingons, Orion slave girls, Leia slave girl outfits…whatever is instantly recognizable and fun seems to aid in producing a successful fan base appeal.