For the Intel Broadwell graphics support under Linux, the kernel-side Intel DRM driver only stabilized with the just-released Linux 3.14 kernel and Intel has more Broadwell improvements for Linux 3.15. Given that this Ubuntu Long Term Support release will certainly see much Broadwell action later this year when these new processors are introduced, Ubuntu kernel developers had to hack-up their own custom Broadwell driver for Linux 3.13 to ensure there will be decent "out of the box" graphics support. This shouldn't come too much as a surprise given for months it's looked like Ubuntu 14.04 LTS would have a 3.13 "franken" kernel.
The i915_bdw driver that has been added to Ubuntu's 3.13 kernel is the Intel DRM driver back-ported from the Linux 3.14 kernel plus the extra changes that the Intel open-source developers have lined up from Linux 3.15. Ubuntu's standard i915 driver has then been modified for the Broadwell PCI IDs to instead load this i915_bdw driver that comes down to just being the newest but back-ported open-source Intel DRM driver code. Other Intel Linux users will continue using the Linux 3.13's Intel DRM driver.
Intel's troubled Sandy Bridge launch for Linux users.
Other graphics drivers didn't receive any special treatment for pumping in new bleeding edge code given this is an Ubuntu LTS release. Generally though the newer the kernel code the better for open-source Nouveau / Radeon / Intel graphics users, especially on the Radeon side right now with many Linux 3.14 improvements. For those Ubuntu Linux users wishing to run the newest code, try using the Ubuntu mainline kernel archive and Oibaf PPA -- those are the two methods we commonly use for fetching the newest kernel and latest user-space Mesa/X.Org drivers when not feeling like building from source.