In addition to the new features and changes described here, there are many bugfixes and interesting improvements, a full list of which can be found in CHANGELOG.md.
There is an entirely new HID (Human Interface Device) implementation: see Working with HID that works cross platform (Linux and OSX thus far). This deprecates the GeneralHID interface. Also the LID interface has been updated to match the API of the new HID implementation.
Apart from UDP, the TCP-protocol is now supported.
When mapping controls of synths to busses, their number of channels is limited to the number of control channels, avoiding a "spill-over" of mappings.
step2shape, which steps to a value at the end of a shape
The following operators have been added as UGens and work the same as in sclang:
While much has improved and many bugs from 3.6 have been fixed, there are still many known issues. For a complete list see: https://github.com/supercollider/supercollider/issues/