Boyer compared the Linux kernels available in Ubuntu 13.04 and Fedora 18 to see how their configurations differed. Both kernels were based upon the upstream Linux 3.8 version.
The Red Hat engineer found that the Linux kernel configurations for the most part were identical but there were some low-level differences (number of supported CPUs, NUMA, timer differences), Fedora using SELinux by default, and Ubuntu is generally enabling a lot of older support/features than what's found in the Fedora Linux kernel.
Josh Boyer confirmed his Ubuntu vs. Fedora Linux kernel configuration with, "There are no shocking differences between the two distribution kernels. The few areas where we differ that might matter are worth reviewing on both sides, but at the end of the day none of those options are going to make any kind of massive improvement. I consider this to be a worthwhile exercise, but one I won't repeat for a while. I can only stare at kernel configs for so long before it makes even bugzilla start looking interesting."
Read his findings in full on his personal blog.