The matter of the default Linux kernel version for Fedora 20 has been brought up multiple times now on the mailing lists and forums. Here's one such instance of Fedora users being confused or disgruntled by no Linux 3.12 by default. Due to the original release schedule and in terms of test coverage, the Linux 3.11 kernel is being used by default in Fedora 20 while the Linux 3.12 kernel will likely be pushed down as a launch-day update to Heisenbug.
Fedora developers generally like to stay on the major kernel version that they are on at the time of issuing the beta -- in this case, Fedora 20 Beta was on Linux 3.11. For those who want to easily upgrade to Linux 3.12 without compiling the kernel yourself, you can already easily get the new kernel using the Fedora Rawhide Kernel No-Debug Repository.
An updated kernel will likely land on launch-day so at least the installation media is shipping with a well-tested and proven kernel should there be any troubles with the new kernel. Fedora at least is liberal with their packaging decisions in that they do allow major kernel upgrades -- and other package version bumps -- over the lifetime of their supported releases. When the exciting Linux 3.13 kernel is stable, you can also expect to find that piped down as another system update.
While Fedora 20 isn't running the latest and greatest kernel by default, at least it manages to have a lot of exciting features for desktop end-users, developers, and server/workstation administrators. Fedora 20 will ship in two weeks unless there is another delay.