Sandboxes have some nice advantages, but I think they would need to be really careful with the implementation. Too little sandbox and they are mostly useless, too much and you end up in the Disneyland-computing that is the iPad.
Besides that, one of the defining things of Windows, for better or worse, is that applications can do anything they want if they dig hard enough and there are lots of legitimate cases for that type of application.
Would most windows users be better served if the system was more like the iPad? IT support seems to think so, and except for developers, this maybe true.
The relevant question is, "How much more like the iPad?"
If you look at the evolution of the sandboxing on the iPhone/iPad, you'll remember that the first version didn't even offer cut-and-paste between applications, and that multi-tasking is just coming now.
