I would just say be warned!!
We stayed at the Poly last year and I had told my family we HAD to dress up, EVERYONE does!
When we trotted out of our hotel room Husband was in a john Belushi style toga outfit, sister was dressed as Cruella,Bro in law as Magnum PI, kids in power rangers and lightening mq queen, and me and my mum wore devil tails and horns - we were quite a sight!! Got plenty of stares as we walked through hotel!
As we approached the monorail queue we expected to meet up with lots of other dressed up people, but no! - OK I thought, wait till we get on the monorail.... but No!! (one boy with a goofy hat?). I was getting serious daggers from all of my family and comments through gritted teeth like "its ok for you, you're not in a bloody toga"
When we filtered out of the monorail queue and joined the masses leading up to the entrance of Magic Kingdom, there were a few kids dressed up, but no one like us motly crew!! Then, thank the heavens, my salvation, a HUGE (and I'm not over exagerating here she was HUGE) woman in a green spandex Tinkerbell costume

.... thats when I knew my family would not kill me!!!
Once we were inside, we did see other families dressed up, but I would say on the whole it was more kids, and when I go again this year I will probably just do what I did last year, accessorise rather than totally dress up.
Of course its up to individual tho, some people don't mind being the centre of attention, Id rather not.