Mesh fill vertices don't seem to be bothered by any snap settings, they don't snap to anything. This is problematic when you try to shape a rectangular mesh fill into a particular shape, like a donut. Snapping feature would make it easier.
Having radial/circular mesh structure as an option beside the rectangular grid would be pretty cool too.
In fact you can't even use the cursor keys to move the mesh vertices like you can do with regular vertices.