A brandnew rendering engine
Making more use of the GPU instead of the CPU
Polygon digitizing
Lift the restriction that layer CRS and project CRS have to match
Geometries will be reprojected into the proper CRS on save
Tabs in forms
Because ordered information is easier to chew.
Prepare them on QGIS desktop with the drag and drop designer.
Lots and lots of code cleanup
You don't really want to know.
But if do you want, check the commit log.