I've been to the GDC (game developers conference) a few times, but I haven't been to a convention before. One of the best parts of GDC was watching a plus sized woman win best costume as Vivi (FF:IX) by quite a margin over the slutty costumes.
Also: meeting Tim Schafer (Lucasarts adventure games, Psychonauts, Brutal Legend, etc...) and attending a keynote by Shigeru Miyamoto (Mario creator)