X.Org Server 1.19 features a few big features including threaded input support that had been in development for several years (more details), PRIME synchronization support as spearheaded by NVIDIA, XWayland pointer confinement and warping and other XWayland improvements, modesetting driver improvements, Windows DRI extension support, GLAMOR improvements, and more.
The XWayland improvements are significant for those wanting to run legacy X11 games on Wayland, GLAMOR and modesetting improvements will affect a lot, the threaded input support is long overdue, PRIME synchronization is big for Optimus/multi-GPU users, and the other improvements all around are nice. There are also many bug-fixes with X.Org Server 1.19
Given that it's been 12 months of development, X.Org Server 1.19 has around 600 changes. This is while X.Org development was much slower in 2016.
X.Org Server 1.19 does break the driver ABIs. NVIDIA has already been working on X.Org Server 1.19 support. On the AMD side, AMDGPU-PRO shouldn't have much of an issue as you can switch to the open-source xf86-video-amdgpu DDX code while those running the legacy fglrx driver releases will be out of luck.