The following list tries to describe all the new features that are already implemented or currently being worked on. I cannot guarantee that I describe everything correctly, since for most items I haven't read the code but only looked at the ChangeLog entry. I can however guarantee that a few things are missing.
Older items are on top, newer ones further down the list.
At the end a few common things are listed. XInput support GIMP does now support special input devices like drawing tablets by default. It depends on the device being supported by your XServer. This is true for example for Wacom drawing tablets. Support for multiple devices, pressure and tilt sensitivity is slowly added to all paint tools.