It's the part that can legally be distributed with Linux distributions (including in-car OS) due the kernel's license. The actual functionality is in the proprietary user space driver

I'm not sure. Didn't they just move the code that was previously executed in the proprietary kernel module to the new also proprietary userspace driver that's just connected to the hardware by this new and open source wrapper module? And the other half into firmware? It's still arbitrary and closed code that gets forwarded to the hardware. And running there it has access to all the memory, screen content etc... I'm not sure if this is a win concerning security. I think it's pretty much unchanged.

But there are several big advantages. Now the kernel probably won't get tainted any longer and we can have signed kernels and activate secure boot easily. And that's maybe a big plus for security. And I hope we'll get the convenience, too. In the past I had the NVidia driver crap out on me while debugging stuff with recent kernel versions or release candidates. And NVidia was lagging behind, leaving me with a console instead of the desktop environment...