MGC is a two day event. One day is all you really need to see everything there, though.
There are speakers (such as the legendary MARTY GOLDBERG), a whole lot of people selling stuff (Warp Zone had the best stuff last year, IMO), a Pinball tournament (and lots of pinball machines on free play), and a game room with lots of rarish games that they swap out throughout the two days. They are also doing some new stuff this year, like a video game tournament. (hello, Kamino!)
I'm sure other people here can tell you what is going on better than I can as they are actually organizing the event.