The reason for going with this extra 3.0 release candidate were several regressions had affected the previous release. Among the fixes that have been pushed into this weekend's version included fixes for UXA, DPMS, TearFree, and other changes. This is the first release in one month so there's a fair amount of work in this latest version done in large part by Chris Wilson at Intel.
The xf86-video-intel 2.99.908 driver information is available via this Git tag. One of the major improvements for the 3.0 driver is the enabling of SNA acceleration support by default in place of UXA acceleration, while it (and GLAMOR) are still around as optional acceleration modes. There was also originally Mir Display Server support, but that ended up being reverted from the mainline Intel X.Org graphics driver.