I was in EPCOT to ring in Y2K and will be back again this year. Get there early, I was surprised at how much I was able to get done. The crowds will be huge, mind boggling huge especially as the day/evening wears on. At one point , in Mexico, we could't move there were so many people going in oppoiste directions, it actually got scary for a few minutes. I made PS for lunch and dinner to be sure I had a place to hide, eat, and rest.
Don't forget, if you like to have a sip of the bubbly for NYE you can't get it at MK.
If you have it your mind that the crowds will be huge, expect to hit some rides early, set up your PS and just enjoy the festive atmosphere you will have a great time. It's all about manageing expectations.