I'm a staff member, so feel free to take my opinions with a grain of salt ^_-.
Anime Vegas has had an attendance size between 3,000 and 4,000 the past few years, IIRC. Most of our attendance comprises of people who live in the Vegas Valley, but there are also a lot of people who come in from Utah and Southern California, especially in 2008 and 2009. It seems to me that most of our attendees are between the ages of 14 and 21.
Of course we have our share of overly hyped up fans that you have to watch out for, but for the most part our attendees are well behaved. (If they're not well behaved tell a staff member! We will take care of the problem because none of us put up with that BS.) We have taken steps to bring the maturity level up, like ban signs and yaoi paddles. I've found that most people at our convention are extremely friendly and approachable too.
When it comes to guests, if you're a fan of American voice actors you cannot pick a better convention! Take a look at the guest list on our website - it's already a mile long! The last few years we've also had a Japanese guest and a few Japanese bands out to play. We will again this year, but none of that it set in stone yet.
We have a wide variety of panels, including the gameshow-type that are so popular nowadays. TBH I'm not much of a panel person unless I'm running it. I am one of the panelists on the Cosplay panel every year, which is usually one of the more popular panels during the weekend. Last year we had a poster gallery in one of our panel rooms which really cut down our panel variety, but this year that will not be happening
Since you're posting this on cosplay.com, I'll assume that you'd be interested in our Cosplay events. We have two this year. First, the Masquerade is so much fun every year! I used to run it and I've been amazed at how much our cosplayers have stepped up their game. Now it is run by Natchan and she is a great organizer! Our other Cosplay event, the Masquerade Ball, takes place the first night of the convention. It's for everyone to have a chance to dress in fancy attire and dance the night away! It's open to all attendees (as long as they're in proper dress) but we also have a mini costume contest for the best formal costumes! There's a thread on this forum about the Ball; please see that thread if you want to know more.
Well, that's all I can think of at the moment. If you have any other questions please feel free to ask!