Good question! The answer however is a bit of a challenge, but I think to sum it up in the simplest way would be all the above.
The People: I would think it would be a given that people would come and go. I personally feel that even though people come and go the atmosphere of fun should still be able to stay the same.
The Threads: Over the past 10 years there have been some wonderful threads on this forum. Some Happy some informative, some sad, and some down right silly and they where all a great thing to be a part of. Sure we could go through and bump some of the old threads up, but the fun would almost seem forced. We need new threads with new ideas, and in a way spark a new energy into the forums.
The Respect: This is one that I have been very passionate, and at the same time very frustrated with. Websites like Toghpigs The Muppetcast, and the Muppet Mindset have grown to huge heights with daily updates, stories, and interviews that Muppet Central has become the black sheep of the Muppet fan community. Which is sad since in it's glory days this site was the only one that was keeping up with updates and so fourth.
All of these things with the exception of the people could be a simple change. It would just take a little bit of TLC.