I went as part of a larger family that included kids, but after the first hour split off with my mom and so we were two adults without costumes and had a blast. Very few had costumes, but it was a few years ago so that may have changed. We went on Halloween night, and I did notice that it was so warm that most kids had pulled off their costumes not too long into the evening (polyster is not good in heat, and kids get "over" the costume thing in the presence of fun rides and just want to be comfortable). So for those who bring costumes for the kids, try to be prepared for a change of plans in case the kid wants to peel it off after an hour!
I much preferred the Halloween Party to the Christmas Party (but both were worth it). Seeing the villains in the parade was a hoot. I found that as adults we got around a little easier than the families with small kids -- we didn't stand in chracter lines, and we didn't bother with trick or treating, though as the night got later and we saw CMs standing around with bags outstretched we'd grab a tootsie roll or whatever on our way to the next attraction.
There was a different feeling in the air, and the later it got, the better -- many of the youngest kids left after the first parade, and the park got a really eerie feel with scary music towards the end.
You won't feel out of place at all!