I went to one a long time back. Twas aight.
First thing you'll notice is the line. Unless you bought tickets in advance, it could take hours to get through the door. Keep in mind this was a small con; I can only imagine what it's like for the huge ones.
The convention complex was linked by a garden (via a pond and a bridge) to a hotel. It was honestly rather picturesque. Here I found people playing Magic, taking pictures of cosplays, and fighting with foam swords (Actual practice swords, mind you. I'd reckon most of us would get "decapitated" in an instant.) This was also where I took a picture of this one dude that happened to cosplay as somebody from an H.P. Lovecraft book. Looking back upon it, I can't help but wonder how he's doing in life.
Eventually you start noticing the bad cosplays. One of my favorites was the chubby-faced Filipino kid dressed up like Black Star from Soul Eater. Unless you're an asshole though, you're not gonna care. As long as they're having fun, what else oughta matter?
An essential piece of advice: Bring money. Lots of money. It's not that the food was particularly expensive (it wasn't), but there's so much awesome merch available on the floor that you'll kick yourself for not bringing more. Posters, T-shirts, wallscrolls, dakimakuras; miniature bookstores stuffed with manga and anime DVDs! For a small two-day con like AnimeNEXT I got by on a hundred-something, but I'd recommend bringing $300+ if you're a richfag like that.
The other notable part of a con (aside from trailers and other official productions) is the panels. Essentially: Voice actors, internet celebs, or even just really good cosplayers get together to talk to fans, answer questions, and crack jokes. I went to a rather entertaining one with Hetalia cosplayers, and another one with these people who made AMVs. My friends didn't enjoy the latter, but I found it funny.
Notably, there are also adult panels, usually held after dark. Seeing as I wasn't 18 or over at the time, I couldn't tell you much about 'em.
Among other things there was the Cosplay Contest, but there was also Cosplay Chess! It's exactly what it sounds like: Two players go head-to-head on a "small" board, while cosplayers (signifying the pieces) move accordingly about a "big" board. They do a little ditty when one piece takes another, which ranges anywhere from epic to downright hilarious, such as when I saw C-3PO punch out Alucard from Hellsing.
Soo... Yeah! That's pretty much my one and only convention experience. I'll leave out the parts where I got separated from my friends, and where I got treated like dog shit when I was with them. In retrospect, I can't help but wonder if it'd have been better to just go by myself.