Its not the entire park thats open for the party. Some attractions are up, E ticket ones, some others may not be. If I'm remembering right, I think I remember 7DMT, PotC, PPF, Little Mermaid, the Carousel all being open, Space Ranger Spin was down, People mover. Not sure about Space Mtn. I dunno if there's a list anywhere but if anyone finds one please post it up here. Waits are pretty short. 7DMT having the longest line of an hour or so by midparty.
People watching with the costumes will be fun. I think you're missing out on the fireworks and meet and greets but thats me. Meet and greets feature normally unavailable charters like the all 7 Dwarves, or Jack Skellington.
You can probably get more rides in for very short waits during the fireworks show, more so that a normal night fireworks show I'd guess.
All the E Tickets are open, as is most everything else. Minor attractions like Carousel of Progress or the Treehouse are closed.