I've always favored Windowmaker for the usability of it. KDE and gnome seem very bloated. Like, "Why don't I just run Microsoft ontop of my Linux OS?" Take two steps back to take one step forward.
WindowMaker is awesome, I like the ability to drop icons into docks ... I also liked the original CDE (which these days is only available on AIX and Solaris anymore) which allowed to drop icons into menus. XFCE claims it's based on CDE, but it lacks that feature (drop works only in a special menu editor).
KDE and GNOME have been reducing some bloat over the years ... and they're using hardware accelerated graphics now if available, so they're much more responsive than they used to be.
On Linux, you can still run WindowMaker if you want to, or any other window manager that you like. Nowadays, there's enough file managers to choose from. (like Thunar from the XFCE project which is pretty good).
XFCE has been created as a light-weight alternative to GNOME, and it does have a bit of CDE feel to it (w/o the constraints). CDE is script-based, and XFCE isn't, which simplifies things greatly.
There's an even more extreme light-weight desktop called L
E which calls itself "so easy your grandma can use it". But last time I checked, it was a work in progress, perhaps they progressed a bit since then.
If you like the classic desktops of Windows 95 and OS/2, you can also use ICEWM, which is very primitive and comes with hundreds of skins (look&feel options).
I like GNOME and KDE ... especially the feature to drop folders into the folder bar in any browser window. This makes it possible to access frequently used folders more quickly.
On slower machines, XFCE is a good choice. But WindowMaker is also pretty good. Both run on pretty low-spec gear.
GNOME is based on CORBA but that is apparently rarely used ... on OS/2, there've been templates which made instance creation very simple. Only few GNOME applications provide that feature.
---------- Post added at 07:45 AM ---------- Previous post was at 05:55 AM ----------
I'm back to GNOME ... can't find a weather plasmoid for KDE that works ... :-/