Pre-emptive multitasking combined with intuitive, flexible and extremely efficient interface?
Taste Workbench.
Taste Amiga.
Well, it's simple, and nice. In a way, the colors aren't great, but there are a lot worse color combos around. Everything's skinned, including the cursors, but loses heavily on originality. It's intuitive and easy to use, but there's nothing about it that really catches the eye.