NYE is one of the most fun days at Universal. Everyone has that energy about them--employees and guests.
After a certain time, the music in the parks changes from its standard fare to a more party-ish kind. There are also DJ booths set up, party favors handed out, etc.
Crowd-wise, it's horrific. NYE is one of the busiest days of the year, with more and more people coming in as the night goes on. Lines stay at a constant 45-90+ min from 11a-11p. The parks usually stay open until 1a. Citywalk has a special live performance. As yaytez said, Cyndi Lauper was there last year.