Hierarchical start menus are also useful, for that matter.
I'm increasingly convinced that overlapping Windows are not particularly useful (they are better than one-thing-maximized-is-the-only-option), but tiled windows are very useful.
One thing that makes overlapping windows infinitely nicer for me is a feature in a lot of Linux WMs where window borders "catch" on other windows and the edge of the screen when being moved or resized. That, and having a key to move windows from anywhere, rather than just the titlebar.
edit: And focus-follows-mouse/x-mouse.
I also like the feature of using the scroll wheel to change window position (above/below), especially if I'm browsing and have a few related windows open on a desktop. I didn't realize how much I missed the feature until I tried Plasma 5 on KDE briefly (it has it, too, but it didn't migrate my configuration).
There's actually a registry setting to enable it on Windows, but so many things are so completely unaware of it in their design that it breaks a lot of stuff.
Edit: To me Metro tablet size without desktop or iOS with tiling would be the best.
However what I dread most are the vendors and their crapware bundeling programs. Uch.