It's more than the difference in user space. Google wrote a bunch of extensions to the standard kernel. The kernel developers consider these either badly implemented or unnecessary.

On top of that, Google presented the extras and said "there you are, they're finished" and refused to make any changes. The usual way to get stuff into the kernel is to work with the kernel team to get the code into a state they consider complete and suitable.

So the stuff was dropped, there is a technical and a political argument and the net result is that 'official' kernel from kernel.org cannot run underneath android.

I don't know how this would affect things like the dalvik VM. A guy at ubuntu made some progress porting the android and dalvik framework to more generic linux systems, but in the end google have still got a lot of closed source stuff on there that makes the system worthwhile - for instance all the stuff that enables login to google services.