I suggest perhaps taking a look at Epcot. Fiance and I went there last New Years Eve (2005), not knowing what to expect. I am a long time Disney veteran, and have been pretty much every time of the year but had never been on New Years Eve.
It was a BLAST! I thought perhaps they would just kind of do it up a little, but they went all out. There were different dance stages in each country, each one playing different music, from rock, to pop, to techno, to salsa, they had music for all kinds. It was definitely full of people, but not so much that you were contstanly running into people. Plus, unlike in MK, you are free to partake in the "drinks" of your choice!
I was so impressed that we will definitely be going back again, that is one of the most fun New Years Eve's I've ever had, I mean the place was just a big massive party, and they had TWO fireworks shows for New Years, one at 9 PM and the other at 11:50 with the countdown!