It’s important to remember that Xubuntu is not XFCE. What I mean by that is, Xubuntu uses XFCE as a desktop environment, but adds a lot more from ordinary Ubuntu that doesn’t technically appear in the XFCE playbook.
By itself, it in its purest form, XFCE is almost as fast (in my experience) as Openbox or Fluxbox or sometimes even IceWM. Xubuntu dumps an immense load on XFCE, and that’s why it doesn’t seem fast on an older machine. And that’s why I don’t endorse it for anything slower than about 1.4Ghz: The bloat overtakes the benefit of using Xubuntu.
Be that as it may, the XFCE community has a clear idea of what system applications are aligned with the desktop’s goal — and they have a great list of applications here. I love finding new application lists; it’s like Christmas, Ramadan and the Lunar New Year all in one.
There are a lot of common programs there that you might recognize. I make a point of using a lot of those, like ePDFView, Zim or even just Sylpheed. There are some new ones there that I want to take a look at, and maybe you might too. I doubt any one of them, except perhaps for some of the core, bundled XFCE tools (like Terminal) will drag in much of the entire desktop if you’re only after the program itself. Watch the dependencies and see what happens.
Personally, I’m going to check out Tasks, Oxalis and maybe see if I can get SLiM running under Crux on the battered Thinkpad. Such fun! :D