you missed a big option in your list.
as much as I detest the bloated thing, the KDE 4 GUI for the open source operating systems was designed to be able to completely REPLACE the standard windows GUI.
so KDE 4, 20 desktops possible, different backgrounds per desktop
preview of the desktops
fast switching [ both hotkey and a task bar icon ]
easy shifting of applications to any desktop
task bar can show all open apps or only from current desktop.
get a copy of kubuntu or pclinuxos or mandriva's live cd all use KDE and you can check it's features out yourself without doing an install.
I wouldn't recommend actually installing KDE 4 on a windows system, their own documentation on doing so is not being maintained and it does say it is a compile from source only, with dependency he|| involved in doing so. :/
