All of the big attractions will be open. Stuff like the Hall of Presidents doesn't run during Grad Nites. Some of the food places won't be open. I'm not sure if all of the gift shops are open, but I do know they have special carts selling Grad Nite exclusive merchandise.
It's a very fun, party atmosphere all night. The concerts and DJs seem to be a lot of fun. I don't know who they have lined up this year, but they usually have somebody pretty cool.
The biggest thing to keep in mind is to have fun while obeying the rules. During the 4 Grad Nites I worked in 2006, we kicked a total of 34 kids out of the park for their actions on Splash Mountain--just our one ride. Imagine the totals for the entire park. Grads get a bad rap because of the obnoxious ones who've done stuff in the past (the year before I worked there, someone had lit Mickey's pants on fire during a meet & greet--not good, even though I'll admit we had a good laugh about it. Don't worry, Mickey's friend was unharmed...). We had to have extra people watching the cameras in tower, and another person who was designated to go inside the ride to track down people who got out of their logs because so many Grads do it. Just stay in your ride vehicle. It's not worth it. (On another amusing note, my friend who worked on Main Street got assigned to "hanky panky patrol" and had to hide in the bushes to make sure that nobody went down there to fool around.)
You're also likely to find that CMs are a little more curt than usual. We were instructed by our management to enforce the rules more strictly, and not take any lip from anyone. If you're caught line jumping or something like that that usually gets brushed off, you're a lot more likely to get yelled at.
Above all, HAVE FUN! It's liable to be the highlight of your year.
