This is a list of all of the new features in paint.net 4.0 that have already been implemented.
I couldn't find a consolidated list of new features in paint.net 4.0, and thought that it might be useful for people who don't want to dig through the blog.
Let me know if I missed anything, and I'll add it in.
Now updated for beta 4.
A small selection of new features:
Greatly improved performance
"Fine-grained" history (Can undo/redo every change, not just those that commit pixels to the layer)
New shapes tool
Live adjustment of many features (tolerance, sample origin, rotation origin, etc)
Full feature list