He may have been joking, but there's some truth to it: checking out people's cosplay and getting pictures is one of the most interesting parts of attending anime conventions. In addition to this, however, there are often interesting panels to attend, perhaps musical performances, screenings of new animes or films, autograph signings, and fun shows. In particular, I love when shows do "Cosplay chess."
There's also the artist alley where you can get beautiful custom artwork that you'd be unable to get anywhere else, the dealers rooms where you might be able to find a deal on some cool stuff. And many cons have a video game room, where you can chill out and have fun. I actually got to play an arcade Shinobi cabinet this past year at NYCC, which was really cool to me.