New Year's Eve is very, very crowded. Epcot usually does not close down, but the MK does during the day. We always go into our park of choice early in the morning before the crowds hit (grab fastpasses for later) and then leave for a mid-day break. We then return around 5:00 - 6:00 and never have a problem returning. If you do not have an AR for your park of choice, do it today! We're at Rose and Crown this time, about an hour before the early IllumiNations fireworks.
The midnight IllumiNations is great!
The only downside is that Epcot can get a little "drunk out" if you know what I mean. Add that to the low lighting and you really need to keep your head up when you walk through the park.