We went to the party on Friday night. DH and I dressed up and frankly I think it made it more fun for us! DD8 was Kit Kittredge, DD6 was a ladybug, I was the ladybug's momma and DH was Moses. DH got a lot of attention as Moses
I went to the first party and it was kind of interesting to see the difference in costuming between WDW's MNSSHP and DLR's MHP. Overall, I would say that fewer adults dressed up, but many of those that did had really good costumes. If you choose to wear a costume, you won't stand out, but by the same token, if you choose not to, you also won't stand out.