Nothing. Fully open CyanogenMod is called Replicant, and just take a look at its support list. That's how open and free Android platforms really are.

http://redmine.replicant.us/projects/replicant/wiki/Replican...

Those look like it's just missing drivers? Android itself doesn't have much to do with that.

Notice that it's still based on 4.2, there are not so many of devices it can run on and even with fully documented and open platforms like GTA04 it's still a lot of work to make it run.

Plus there's absolutely no way to upstream their work, so it's a massive maintenance burden (one of the reasons they're still on 4.2)

AOSP is just what zurn said above - a code drop. If Android was really an open platform, Replicant either wouldn't be needed or Replicant devs would have much less work to do. Openness of Android is great for Chinese vendors making cheap tablets, so they have a ready system to put on them for almost free - but it's not so good as an open platform.