The big feature of libinput 1.6 is the new touchpad acceleration code that makes it "no longer terrible." This touchpad acceleration code is much improved over the earlier code and should address complaints that it was too slow previously.
Libinput 1.6 has also changed around tap parameters so that they provide for a more responsive cursor. There have also been many bug-fixes and device-specific quirks added to libinput 1.6 development.
The libinput 1.5.901 development release available today will serve as the testing grounds prior to Peter's plans of officially releasing v1.6.0 in about one week. The complete list of changes for this libinput 1.6 RC can be found via this wayland-devel message.