I've never been for New Year's Eve but I have read that it is the busiest/craziest day/night to be in the parks. They reach max and gates are shut down. I've seen pics of Epcot during New Year's Eve day and it is wall to wall people.
I just checked allearsnet and this year MVMCP only goes thru Dec 18. But Epcot's Candlelight Processional goes thru December 30 and over at the Studios the Osborne Dancing Lights (fantastic!) go thru Jan 3.