There simply aren't too many regression fixes going into the Linux 2.6.39 kernel and it's simply not because all of the developers are on holiday.
Linux 2.6.39-rc2 is mostly comprised of architecture updates (ARM), various driver fixes, and other random fixes. It's "a small -rc."
It's release announcement can be read at LKML.org.
While this release candidate is not too exciting, the Linux 2.6.39 kernel introduces some exciting changes for new drivers, open-source graphics DRM updates, etc. At the time though, we're already looking forward to the Linux 2.6.40 kernel months later.