In the 2.6.38 release announcement Torvalds shares some of his favorite work in this new kernel. "As to the "big picture", ie all the changes since 2.6.37, my personal favorite remains the VFS name lookup changes. They did end up causing some breakage, and Al has made it clear that he wants more cleanups, but on the whole I think it was surprisingly smooth. I think we had more problems with random other components (nasty memory corruption in networking etc) than with the rather fundamental path lookup change."
The Linux 2.6.38 kernel carries AMD Radeon HD 6000 series Linux open-source support, Fusion Linux support, Radeon KMS page-flipping (a huge performance win!), Nouveau GeForce 400 / 500 acceleration support (still requires external non-free firmware), and many other open-source graphics improvements.
Beyond the graphics, there is the kernel patch that does wonders, other low-level improvements, Btrfs LZO compression, transparent huge pages, new drivers, and much more. Some of the other Linux 2.6.38 changes that fall outside of our main focus at Phoronix are talked about at KernelNewbies.org.