What is XFCE still needing to become the #1 choice
Although I have some doubts that XFCE is "so very much lighter" than GNOME (GNOME 2.20 doesn't take too much memory if you don't start all kind of crap), it is still lighter, and in a few years there will be less and less antiquated computers who require extra-light window managers (Fluxbox, Openbox, Blackbox, WindowMaker, IceWM).
So far, the applications, tools, accessories and applets that come with XFCE are not contaminated with the Mono virus, and hopefully this won't change very soon. (Or at least, I hope so.) XFCE is reasonably mature, and constantly improving, so it has all the chances to become the mid-weighted Desktop Environment of choice pretty soon!
To make XFCE be #1 (in "market share", if you want to put it this way), it doesn't have to look like Vista, nor to look like OS X Leopard; it's good that it's that configurable, but it's also good that it's simple by default.
What is XFCE needing to reach the Nirvana? Here's the way I see things.