There is a significant amount of people unhappy with the direction of GNOME 3 who do not enjoy KDE and find LXDE too weak in features or who just like the look and feel of the old GNOME 2 desktop. Here are their options in Fedora 18, as they can be seen in the recently released Beta.
Xfce is by now an old desktop environment, with years of development and an existing community. When GNOME changed drastically, it was seen by many as the de facto replacement (before, it was seen by one as "GNOME Lite"), an important number of users adopted it, even Linus Torvalds used Xfce for a while.
rest here