You don't need to arrive early just make sure you have priority seating at one of the Magic Kingdom restaurants. We had 7pm reservations at Liberty Tree Inn and we got to the park around 6pm. I brought the reservation confirmation with me in case there was a problem getting into the park but they let us right in without question. The park was jammed and they wisked everyone coming into the park behind Main Street (with lots of security) directly to Tomorrowland.
We knew it was futile to try to get onto any rides so we made our way to Frontierland and got to the restaurant in time to give them our name. The wait at the restaurant was probably over an hour but we had a good time. The dinner was terrific and we got hugs from Minnie, Pluto, Chip & Dale. What a nice way to end the year.
Main Street was insane and my friend and I were both widow's who just wanted New Year's to pass so we just sat down and braced ourselves for midnight (it's the hardest time when the one you love and want to be with at the strike of midnight is gone). We sat on a bench in Tomorrowland under the Tomorrowland Transit Authority and listened to the recording of the ride. Suddenly fireworks started and for about 20 minutes the entire sky was lit with the most spectactular fireworks I've ever seen. We just sat with our mouths hanging open. Then it was over. No countdown to midnight, no Ole Lang Syne, it was just over and not a tear for either of us. I'm sure you could hear the countdown on Main Street but all we heard was the ride recording which was perfect.
When it was over, we went to Fantasyland and went on a few rides. While we walked down Main Street (with a couple of hundred other people), we saw that it was snowing and there were snow flakes all over the castle. It was so wonderful.
Plan on a very long trip back to your hotel, but it was a memory I'll never forget. Have a wonderful time