I like having no clutter on my desktop. By clutter, I mean "anything that isn't a window".
I also prefer to use command line tools through an rxvt shell... so all the gui stuff in KDE and Gnome is just a lot of bloat for me.
I used WindowMaker for a LONG time and loved it... but those icons were just so damn ugly. When I tried Gentoo, I noticed a lot of people raving about Fluxbox, so I tried that out, and fell in love.
Tabbed windows are just great. You can group any windows you want. I set up some keyboard shortcuts for cycling through the tabbed windows in the group I have choosen, and it's just so usefull.
Also, I can use my favorite WindowMaker doc apps. I've got them docked in the upper left corner of the screen with auto-hide on. I really only use Volume.app though, to easily change the volume in fluxbox.
I also love being able to right click anywhere on the desktop to get to the root menu, and middle click to get to the taskbar-like menu. I find it so much quicker to use than a staticly-placed start menu and taskbar.
The only things I missed from WindowMaker were being able to use "%s" in my menu to insert selected text in to it and "%a" in my menu to pop up an input box and then enter that text into it... (i.e. "phoenix %s" would open phoenix with whatever I have selected with my mouse where %s is... GREAT for centericq. and "phoenix %a(Enter Url)" would pop up an fbrun box asking for a url (with it's OWN history list!).) So I hacked the source code and made a couple patch files to add that functionality into fluxbox. I couldn't be more happier with any other WM. :)
Actually... I think I might like
ratpoison. It's based off GNU Screen... I've been wanting to try it for a while now.