Dragoncon is All The Things. It is intense, it is incredible, it is everything. I've been going since 2002, I think. 5 days of an 80,000-strong geekgasm. Nerdigras. Comic-con is bigger, at 300,000, but has two major differences. 1. Comic-con is industry-run, Dragoncon is fan-run. 2. Comic-con is in a convention center, which means it shuts down at 6pm, and everyone scatters to various hotels around the city. Dragoncon is held AT the 5 downtown hotels in Atlanta, all right next to each other, so the party is 24/7.
There are about 40 'tracks'. Tracks are fandoms, like Alternate History (includes Steampunk), Whedonvers, Star Trek, British SciFi, Star Wars, Wheel of Time, Dragonriders of Pern, Tolkein, Paranormal, Space Science, Art, Costuming, etc. Each track is running panels through the entire Con. For instance, the British Scifi might hold fan a panel during the day about New Dr. Who vs. Classic Dr. Who, or the Science of Daleks, and perhaps a recap of Orphan Black, and then run screenings of classic Monty Python at night.
There are Miss Klingon Beauty Pagaents, a Rocky Horror Picture Show show, concerts (Cruxshadows and Emerald Rose are usually there), etc. There is a huge art show (over 250 artists), a comic book art show, a HUGE dealer's area in the Americasmart down the street (prob 400+ vendors?), two exhibition halls, a Walk of Fame where you can meet and greet the stars, signings, readings, mega-panels with the big stars (William Shatner and Leonard Nimoy were at one two years ago and it was great!).
You can meet the authors as well. I met Jim Butcher last year, and have met Anne McCaffrey several times (even did a writing workshop with her son, Todd McCaffrey), ran into Diana Gabaldon, literally, as she came off an elevator, and helped her find her hotel, even met Betty Ballantine (started Ballantine books). Stars or all around. I've met Adrian Paul (Highlander), Ioan Gryffud, Edward James Olmos, Michael Dorn, Malcolm McDowell, George Takei, Sylvester McCoy, Jewel Staite, Ron Glass, Morena Baccarin, Nichelle Nichols, John Delancie, David Carradyne, lots of others I can't think of now.