This release candidate contains mostly bug-fixes and other work as Keith is trying to keep X.Org developers from committing any new code that doesn't deal with correcting the outstanding bugs. For the X.Org Server 1.9 blocker bug there are currently 17 outstanding bugs that need to be addressed.
Today's 1.9 RC5 release announcement can be found on the announcement mailing list. While X.Org Server 1.9 doesn't bring as many features or major changes as some of the past releases, this time around it's on a tighter release schedule than normal, but there is RandR 1.4 and some other enhancements. X.Org Server 1.9.0 is expected for release in mid-August.