In the past mainly just Asia and Dinoland are open. In addition to the rides, there are a couple shops open and food is available for purchase. I guess what makes it a "party" is that there's a DJ playing music that's pumped through the parts of the park that are open.
Personally I enjoyed being in Animal Kingdom at night. The view from the first row of Expedition Everest is awesome! But if the party wasn't included in the registration fee, I probably wouldn't pay extra for it.