I have trouble describing it. I think the difficulty arises because at any given point there is far more going on then any one person can adequately comprehend. Here's the best vignette of ten minutes I can give:
At 11:55 am Friday Morning THomas was busy pitching his idea for a new comic book series to the CEO of Viper comics, while Sarah was waiting in line to get the exclusive Harry Potter bag from Warner Brothers, while Martin was flipping through stacks of vintage comic books, while Christobell was demoing the Microsoft game, while James Cameron was screening clips of Avatar, while the Hulk was signing autographs, while Naruto and Sasuke cosplayers were posing for a picture, while hundreds of fiercely independent artists hawked their work, while 'Jane' checked to see if any of her work had gotten bids in the art show, while I was captivated by wooden robots that tickled some long dormant corner of of my brain, while some five year old was bought an action figure whilewhilewhile... so forth and so on.
I a guess a good enough analogue would be to turn a vegas casino floor over to a bunch of street merchants and multimedia conglomerates and fill it with the geekiest, smelliest, most interesting or fun people you know. And then shake it all together like a mason jar full of bugs you're trying to encourage to battle. I am still faltering, still failing to impress on you just how BIG and LOUD and AWESOME and OVERWHELMING it all is.
My advice, to anyone who will listen actually, is to go. At least for one day, at least once. I guarantee you that no matter where your particular interests lay, you will find enough to fill your time there three times over. Unless you don't like fun or art or media of any kind. But then you have issues that I can't even begin to discuss.
So here's what I remember from my adventures at Comic-con:
I remember buying awesome gifts for some people I care about.
I remember my feet hurting something fierce.
I remember all the artists who were nice enough to give me a quick sketch.
I remember art and noise and wonder pouring over my face like molten skittles.
I remember the costumes!
I remember my art selling.
I remember, and still bask in the afterglow of a million varieties of inspiration.
I remember the serendipitous discovery.
I remember the panels we meant to attend (Networking, the Drawn Together Movie, How to Draw Monsters and the utterly disappointing Full Time Output on a Part Time Schedule)
I remember the panels we stumbled into (John K. and Animation on a Shoestring)
I remember the independent films.
I remember lunch with Isaac.
I remember being pitched by Jerry, and vowing to take him up on the offer.
I remember the bags full of schwag.
I remember each and every exclusive something or other we acquired with dreams of profit dancing in our heads.
But most of all I remember the feeling of the world transformed, of wandering about in a daze as a little section of my city was transformed into a fantastic place place where gratis stuff, free hugs and uninhibited geeks flowed like so much wine.
Seriously, just go. One time. You'll see what I am talking about.
(potential followup posts: Comiccon Schwag, making money at Comiccon, What I learned about networking at Comiccon, The Awesome Sketches I got at Comiccon)