The official rules which I posted in the Security thread a few months back which you can find here http://forum.parkfans.net/thread-2322.html?highlight=security but, Costumes and Masks are prohibited, face paint is allowed due to the fact the park will paint your face inside so you might as well paint your face before you go. They may make exceptions for children who appear under 12 years old for costumes, but absolutely no mask for anyone and it is a Class 6 Felony in the State of Virginia to wear a mask out in public if you are over the age of 16. Some exceptions do apply within the law, but generally just don't do it because they don't want people to conceal their identity.
I will say there is great debate as what constitutes as a costume and basically it is up the the discretion of the officer on duty to determine if they think you are wearing a costume. Some people slip through; however, most of the major noticeable ones don't. I have heard of people wearing super hero shirts with capes told that they can not wear a cape because the cape makes it a costume.
Personally, I don't understand why anyone would wear a costume period just because it doesn't make sense. Its not a costume party, you're not trick or treating, thats like dressing up as Santa in the middle of July for no apparent reason.
There are actual safety reasons behind no costumes, you may blend in too well with the scare actors and get punched, creating a liability or you may encourage another person to dress like a scare actor and create a liability. But those are just a few reasons.