The North Carolina Gamedays have been founded on two principles:
1) Meeting in person the people you speak with on a daily or weekly basis, but never get to see;
2) Playing games you otherwise never get to play.
The socializing is a great aspect - it's the reason Rel planned TWO hugely successful dinners this last Gameday instead of one. We meet, talk about campaigns, life, the universe, and everything else (so to speak).
I'm guessing from talking with many of the participants that we all have regular games, but those regulars also are preferential to certain game systems, and are hard-influenced to have one-offs of systems they don't like. I'm a firm believer that if you are motivated enough to be a regular poster of ENworld and enough to go to a gameday, you are also such an avid gamer that you are willing to try almost any system once.
For me, I've run d20 Modern, Mutants and Masterminds, and Arcana Unearthed; these are systems I'd otherwise rarely or never run. I've devoted myself to trying at least one new system or concept every gameday, and the rewards have been phenomenal. If it weren't for Gameday, I'd still not know the joys of Feng Shui, or of playing an Orc and seeing how the other half lives.
Those are what brings me out: Variety of games offered, and the chance to schmooze.

The shopping aspect was pretty nice that Trinocon/GAMER offered, but not the most drawing aspect; I can shop anywhere, but I can't get that gaming and fellowship.