I was at MGM on Dec 31, 2000. We had wanted to go to MK that day, but I was working during the day, and by the time we arrived, we were turned away from the parking lot. By the time we got to MGM, they weren't even charging people to park and enter, because they were already pulling barriers over to prevent additional cars from coming in. We were in the last few cars to be permitted in, at around 7pm.
They had concerts going on. I don't recall who they were. One of them was a contemporary band I had heard of and was at least vaguely familiar with. I didn't think I knew them, but as we walked by the Indiana Jones attraction where they were playing, I recognized the song. There was another band set up in front of the Hunchback theater (I miss that show!), facing out on the street, and a huge street party was going on.
It was cold out. Don't think that just because you are in Florida, you're going to have a balmy New Year's Eve. It was mid 40s, and a lot of people weren't dressed for it. I bet they sold a lot of sweatshirts that day!
Lines for all attractions were very long that night, since the park was at capacity.
We watched the Fantasy in the Sky fireworks from Hollywood Boulevard. The fireworks aren't "special" for the holiday. It's just special that they put on Fantasy in the Sky, since they show it so rarely these days now that Fantasmic is on. The best thing about where we were (other than being an advantageous place to exit after midnight) was that we were able to turn around and see the fireworks from Epcot. Epcot puts on an incredible display in addition to IllumiNations (after the regular show), and it was great to get two shows for one.
One more thing... traffic afterward is a major issue. They did an unblievable job of funeling everyone out. But you go the way they want you to (one route only, no doubling back or going anywhere else). We saw lots of people desparately trying to turn and go a different way only to be turned back by CMs who were no doubt trying to deal with three parks emptying out at the exact same time. That night, at least, all cars leaving MGM were sent out past what was obviously a construction site (now Pop Century), and we were quite lost until we followed cars off property and wandered around a bit before finding ourselves at 192. So bring a map. If you are staying on site, I'm not sure you would be able to drive back to wherever you are staying, so you probably ought to use Disney transportation.
Whew, that was long. If you want to know anything else, feel free to PM me.
