As someone who's a multi-year veteran of both, I describe GenCon as a gaming convention, with costuming and celebrities on the side, at maybe a 4:1 ratio, and I describe Dragon*Con as, functionally, the opposite. I'm the weirdo of the Cobra 21st Infantry --
http://www.cobracares.com/index.htm -- because I actually go
play the RPGs and wargames and stuff. A large percentage of D*Con's population just go there to mill around, look and (and show off) costumes, and sit in on the occasional panel. And get very drunk. The whole time. Like, a buddy of mine walks around with a "mobile bar" backpack full of colorfully mixed drinks and just sprays alcoholic beverages into anyone who asks for some,
drunk. It's basically one giant costuming party with a few other things going on in a basement somewhere.
While it's (theoretically) possible to tell someone about all the cool games they played and stuff, that's such a small part of the D*Con experience that the best answer is, in my opinion a Google Image search. It's a little like Mardi Gras that way. You still won't get ALL the fun, but at least you'll get to see some fun...uhh...costumes.
Both D*Con and GenCon together, just a few weeks apart? Makes for an awesome end to summer break, by the way.
