I haven't been to MGM for NYE in over 3 years, but when I did they had mini dance parties set up around the park with DJ's and/or live bands. One was at the Indy stunt show, another was inside Playhouse Disney's building, another was held somewhere back on New York Street and another in front of the Sorcery Hat on Hollywood Blvd. There were extra showings of Fantasmic that night and then the fireworks at midnight which were pretty good imo. As with the other parks, they hand out free hats and noise-makers. In fact, I plan on spending NYE at MGM this year again. I've been at Epcot for the past 3 years which has been awesome each year, but I'm ready to do something different this year.
I've heard the MK is miserable on NYE because of the crowds. Of course the other parks are busy, but MK always closes to capacity earlier on in the day. As mentioned above, you can see MK's NYE show on the 30th as well. I plan on seeing that show on the 30th myself and then heading to MGM on the 31st.