I have been to NoJ the past 4 years and we enjoy it. Friday night is usually less crowded than Saturday. I would describe neither as packed.
There are three stages with concerts going on simultaneously. The castle stage has the headliners. The crowd during the show fills up the hub area. There is typically controlled access to the front of the stage. (Only so many are let in up there). Most are standing. It is hard to find a good view that also has a seat. The shows last about 45 min to an hour.
The other two stages are in Tomorrowland. One is between Buzz and CoP, the other is that stage between Astro Orbitor and the Space Mt. Bathrooms.
You can get into the MK as early as 4pm with your NoJ ticket. At that time, all rides are open. At 7 pm, only the select rides are open. As I recall Jungle Cruise, does not run during NoJ, but most rides are open. Last year it seemed that they were closing a lot of the rides during the evening temporarily, which was frustrating.
No table service is open. Ray's and Bill's is open. Harbor House is closed, I believe.
We usually arrive early and try and ride rides before the shows start. Then between shows catch a ride or get a bite. I enjoy being around young people, youth gruops etc, so I like the crowd atmosphere. There are college aged kids and parental types there as well. There are a few vendors selling t shirts, cd's, etc. There is always a charity-sponsor raising money for a cause, like hunger abuse, etc.
If you have other questions, feel free to ask. For the first time, I seriously considered going to Rock the Universe this year, but we decided to stay with NoJ afterall.