Check out the changelog in the news article for 4.0 final release. It covers a quick rundown of most of the bug fixes, changes & new features. Quite a lot of stuff has been added & more is getting added over time.
I can't offer much help on the 3d side of the engine I'm afraid. Only the non-3d stuff & lua script.
If you were to use 3d models instead of exporting them from blender as png frames to add as a 2d animation then your game would be 2.5D (3d characters with 2d backgrounds). Apparently the 3d character model system is working a lot better than in the beta versions but I think there's still a few bugs to iron out.
There's a bit of information
here on the wiki about 3d models, but it's mostly gibberish to me!
