Dan,
Honestly, I don't think the UI could be any easier. It seems very simple to me.
I think it is a great layout.
....in reference to other peoples comments about placing objects, maybe you could make some type of 2D screen for placing objects. ...just have 2 windows, one would be in the X,Y direction, and the other would be in the Z direction....then you could drag your object around in both windows, and place it where you want, in reference to the parent object that you would pick in advance (which would also show up in the windows)........you could have some type of elastic measurement lines that would be connected to the parent object and your object, to show your distances in AU....or whatever.
It may make it simpler for some people to place an object just where they want it.