Adding the ability to nest spaces would be a great thing for us. We want each of our consulting projects to have its own space (so it can have its own blog, namespace, permissions etc.) but we don't want to have hundreds of spaces when you look at the list.
It would also allow us to give administrative rights for broad areas to different people (e.g. Bob can administrate the "Projects" space, and all the spaces under it, but he can't administrate the "Sales" space).
It could possibly be used as a way to give personal spaces also - having a "Personal" parent space, with everybody's individual spaces underneath that organised however necessary (with us it would probably be by office location).