X.Org Server 1.15 is the first major update since the 1.14 release in March, which was also delayed. Landing in the past few months after extending the release cycle for 1.15 was the Direct Rendering Infrastructure 3 (the DRI3 story) with the Present Extension, the GLX code re-write, Mesa support changes, and many other features that have already been extensively covered on Phoronix in their own individual articles.
X.Org Server 1.15 is codenamed "Egg Nog" by the ongoing xorg-server release manager Keith Packard of Intel. The next major release of the X.Org Server, version 1.16, will likely occur sometime around the middle of 2014 pending any major delays or release changes.
If you're a user of the NVIDIA binary Linux graphics driver, the NVIDIA drivers were already updated -- even the legacy driver -- for supporting X.Org Server 1.15. The latest open-source DDX drivers shouldn't have any issues. AMD Catalyst driver users may run into problems with xorg-server 1.15; I haven't tried it myself yet nor heard from anyone who has and haven't heard of any official ABI support coming out of AMD for this version.
Disappointing some users about this 1.15 release is the fact that XWayland hasn't been merged (nor Canonical's XMir) as the X11 compatibility layer for having a root-less X.Org Server on these next-generation display servers. For now the Wayland and Mir developers are carrying their own out-of-tree patches against the xorg-server code-base.
The brief release announcement for X.Org Server 1.15 (that doesn't contain any real information for end-users besides download links) can be found on the X.Org mailing list. Over the last 1.15 RC, there's just been a couple more Present Extension patches merged.