Let's face it: Gnome and KDE are fat. Full of wonderful functionality, stable, handy, cush; and big and slow. Use them if you have the resources and the time. I'm leaning slightly toward KDE myself at the moment but that's a religious war for another article. The point of this one is: lightweight and adequate. TWM just doesn't cut it. Most of it's derivatives have gotten big. FVWM and XFCE are two that really haven't. Object oriented, minimalist design, you can take a look at their homepages to get the developer's answers. I'm just going to give you some pros and cons of using them from my experience.
In fact, at this very moment you can endure my note-taking for being an early reader. Who comes to this site, really? I was using XFCE on RH and FVWM on Ubuntu so bear with me while I cross-compare...
XFCE: taskbar