One of the biggest downfalls of major third-party ROMs is that often they miss out on features added by manufacturers. OpenDESIGN is an attempt to rectify this problem, spearheaded by XpLoDWilD of TeamHacksung, a subgroup of the CyanogenMod team. Ultimately, the goal is to rebuild popular and useful manufacturer features and build them into CM9 for all to be able to use.
The site's FAQ says that these features will be "written from the ground up and will be opensourced early on." This should be extremely helpful for the community as a whole. From features like stop watches to S Pen functionality, the biggest reason to continue using a manufacturer skin instead of an AOSP-based ROM is extra features added by the manufacturer.