For 3D, the return of in-engine inverse kinematic nodes (SkeletonModifier3D descendants) has been a big deal for me.

The old 3.x system was a mess, so it was dropped in 4.0. The community filled a lot of the gaps in the meantime, but having the new system (in-engine) is great.

I've been working with it for a few weeks from git master, and everything just works as I expect. It definitely needs some more user-level documentation, though: if I hadn't been following development, I don't think I'd know what everything does, and if someone were new to IK generally, it'd be hard to get off of the ground.