Unity 2D -- the low-powered version of Ubuntu's Unity desktop that doesn't require 3D/OpenGL acceleration and doesn't use Compiz -- is being dropped. Going forward will just be one unified Unity desktop without a 2D/non-composited flavor. Back in May from UDS-Oakland I mentioned Ubuntu 12.10 was trying to kill Unity 2D.
If you don't have any 3D/OpenGL driver, LLVMpipe will be used for accelerating the graphics operations on the CPU using LLVM. Fedora has already been using LLVMpipe for accelerating the GNOME Shell in situations where no accelerated graphics driver is available -- it works generally well, especially for x86 hardware. For ARM hardware without graphics drivers, the LLVMpipe performance for a composited desktop is more sluggish but still fairly usable.
LLVMpipe really does best with multi-core x86_64 processors supporting SSE4 and other modern instruction sets supported by LLVM. Hopefully this move to use Unity-over-LLVMpipe as the default fallback will help encourage Canonical to finally recommend 64-bit Ubuntu by default instead of 32-bit for performance improvements among other benefits.
Of course, the doing away with Unity 2D now demands new benchmarks to be conducted at Phoronix to see how the performance impact is -- since Unity and Compiz have caused performance differences in the past -- and to see how a system's power consumption is affected using Unity with LLVMpipe instead of Unity 2D. Stay tuned.
The dropping of Ubuntu's Unity 2D was confirmed in this bug report.