People forget that a desktop manager should be completely invisible.
You buy an OS to run programs on. You don't buy it to coo at the pretty sliding animations, wow at the alpha-fading of the windows, and be astonished at the "take over everything you want to do" uncloseable windows.
A desktop manager should be functional and utilitarian.
I would argue that we haven't seen a really decent one since the days of Windows Program Manager, and even that's only allowed because *at the time* it was amazingly functional.