A bit of everything:

-IMA-ADPCM support for samples, this means that sound effects can be compressed and use 4 timess less RAM.
-New 3D import workflow based on Wavefront OBJ. Import single objects as mesh resources instead of full scenes. Many people prefers to work this way. Just like the rest of the imported resources, these are updated in realtime if modified externally.
-Mesh resources now support naming surfaces. This helps reimporting to identify which user-created materials must be kept.
-Several fixes and improvements to SurfaceTool.
-Anti Aliasing added to WorldEnvironment effects (using FXAA)
-2D Physics bodies (RigidBody, KinematicBody, etc), Raycasts, Tilemap, etc support collision layers. This makes easy to group which objects collide against which.
-2D Trigger shapes can now also trigger collision reporting in other 2D bodies (it used to be in Area2D before)
-Viewport render target textures can now be filtered.
-Few fixes in GDscript make it easier to work with static functions and class members.
-Several and many bugfixes.