Hey - I can't speak from experience (other than doing DLRP at New Year) but thought I'd chip in anyway!!
NYE is likely one of, if not the busiest days on WDW property. If you are wanting to eat at any sit down venue or stay on WDW property, I would get it booked as soon as you can. I don't know if all the parks will be open for actual New Year. I would be confident that AK wouldn't and that MK certainly would - not sure about MGM and Epcot.....depending on this and your choice of where you want to spend the actual New Year countdown, make sure you get to that park at a decent time during the day as there is always the strong possibility they would reach capacity and you wouldn't get in. You also want to think about where to sit/stand and bear in mind if you do want to be at the heart of things, you'll need to stake out a place well in advance of when you normally would. Also, if you have little uns in tow, bear in mind just how busy it is likely to be - Paris was unreal for New Year and I can't imagine MK being any different. As I say, much will depend on the parks which are open and your own choices. Despite the potential crowds etc, I imagine it is a magical experience.
I am sure someone with more info will be able to help.