So which side are you supposed to pick if you don't like dressing up for anything?
On one hand, fancy parties can be ultra elitist and snobby and I'm really not a big fan of elitism. On the other hand, I've definitely been to more fancy parties than costume ones and actually have stuff to wear to one (sort of).
On one hand, costume parties are lighthearted and don't require a big budget or knowing etiquette rules and stuff like that. On the other hand, dressing up as something or someone is embarrassing, costume parties for adults often involve excessive amounts of alcohol, and they can be just as much about social cliques as fancy parties are. Also, there's always the possibility that someone could come to one dressed as Shrek, though I'm not sure if that's a good thing or a bad thing...
It turns out that going through the pros and cons hasn't really helped me decide...