I like my "desktop manager" to.. not have a desktop (I have a desktop background image, no icons, no menus (that are always visible)) and my window management to be as simple and automatic as possible. I also want the mouse to be 100% optional. Its nice to use it when I'm pointing and clicking anyway, but when I'm not, eg, when I'm coding or in a terminal, I want to be able to control everything without leaving the main keyboard. As such, my window manager of choice is, of course, a tiling window manager (musca), and it is truely lightweight - a few K lines of C code, a few KB compiled binary, very fast, very low memory usage. Hell, it delegates its menu to dmenu (which I find to be an awesome little program - useful, very fast, tiny, easy to use and no dependencies besides xlib).

I run Musca on an Arch install (on an eee PC) for true lightweight minimalism :)