This first post-holiday kernel update in 2013 has various changes from GPU driver work to architecture improvements to file-system changes and various other regression fixes in kernel-land. Linus Torvalds characterizes this week's release as "another week, another -rc. A fairly normal-sized one."
Linus Torvalds has already expressed hope that Linux 3.8-rc4 will be more calm than this week's kernel release. The fairly simple kernel release announcement for Linux 3.8-rc3 can be found on the LKML list.
The prominent changes to the Linux 3.8 kernel release are covered in full via this article.