We're still months away from seeing Broadwell processors make their debut but when they do the open-source support should be greatly, including for the more powerful HD Graphics "Gen8" that come with this Haswell successor.
The latest feature to point out that's now landed in Mesa mainline Git for the Mesa 10.2 release in about three months is multi-sample anti-aliasing support. The Broadwell MSAA support now advertises 2x, 4x, and 8x multi-samples, similar to earlier generations of Intel HD Graphics hardware on Linux with Mesa.
The change was made with this Git commit to the "i965" DRI driver. Unfortunately we still haven't seen any other advanced anti-aliasing modes implemented by Mesa and their drivers besides the long-standing MSAA and MLAA (Morphological Anti-Aliasing).
Hitting Mesa besides the MSAA Broadwell change were also other Broadwell commits.