Crowded. Very very very crowded. It's definitely not a place for someone with a fear of tight spaces or that have issues standing in lines. I have social anxiety issues but they're not normally too bad, I can keep them mostly under control, yet I can't spend more than a few minutes on the sales floor because it just gets too crowded. We're talking shoulder to shoulder, no room to yourself, have to push people to get through sometimes, crowded. But the cosplays are amazing! I admit I go more for the pop culture aspect of the con than the comic aspect, but just being surrounded by like minded nerds is also a lot of fun. Being one of the first to get behind the scenes info or hear about new things coming out is cool.
There's something for everyone between the events outside that you don't need a badge for and the events inside. If you have facebook or Twitter search for the Unofficial San Diego Comic Con blog. They're really great about keeping people up to date with information and it's usually very accurate despite them being unofficial. They're my go-to for source before the con then during the con some of the bigger rooms I usually try to get into have their own twitters to help people keep informed of line conditions.
This would've been my 6th year attending as a badge holder, 7th in general since I went down one year when I couldn't get a badge and just enjoyed the off site stuff.