PAX last year was my first US con and my first PAX, and I went to Prime in Seattle as well. Headed down to Boston from Toronto for East 2013, and I'll be wearing Dante from DmC, and maybe Shepard armor.
It's really different from the usual conventions, there is no dealer room. There expo hall is huge, and does have some places to buy things, but no row upon row of privte shops. Maybe some novelty t-shirt booths scattered about, mixed in with the big gaming demos. There's a lot of rooms for panels, and QAs and previews and whatnot. And a lot of rooms for board/PnP games where you can just pick up and play, and learn with people. It's kind of an amazing time, really different than the usual conventions I've always gone too. And the crowd is generally nicer. PAX has -very- strict policies concerning dress code and behaviour.