When Vertex Shader is toggled on and off, the clouds change position. Also the night texture gets brighter with it on and dimmer with it off. Nothing big, but...
I wanted to test if you'd implemented the option to have bump maps and specular at the same time. So, I added a bump to earth and loaded Celestia - specular showed up, not sure about bump, but anyway, the clouds were black. I tried it on Pluto with bump, without specular, and the test clouds I added were the right colour. So bump and specular together make clouds go black.
For some reason, the Earth looks more round.
Comets look pretty cool actually. Can you add a option to make the blue wireframe fade through different colours and add disco lights?
There might be more, who knows?