It changes on a regular basis. The Dev Server is where we roll out new features, both small and large, for testing before we put them on the main site. So it's more "bleeding edge." Then as those features are completed, we move most of them to the main site. Some features are only on Dev for a week or two before they move to main (small ones), while others (like Dynamic Lighting) that are more major may be on Dev for a month or more before we feel they're ready for main.
Off the top of my head, right now on Dev there are:
- Token nameplates
- Global macros (e.g. ability to create macros by the GM and assign them to players)
- Copy/paste fixes
- Dynamic Lighting
But again, the list frequently changes as we work on different things. I wouldn't think of getting a Mentor account to use Dev just due to a specific feature list (unless it's a major one like Dyn Lighting, maybe). I'd think of it more as getting inside access to what's being developed. We also solicit feedback from the Mentors regarding the Dev Server features, and especially in that early stage where we're heavily developing them, they get to have a lot of influence into how those features turn out.