I couldn't find a standalone application/script that implements freedesktop compliant (XDG based) autostarting of applications, so I decided to write my own.
The project is at http://github.com/Dieterbe/dautostart .
Right now, all the basics seem to work (except "Autostart Of Applications After Mount" of the spec).
It's probably not bugfree. I hacked it together in a few hours (but it works for me :-). Bugreports welcome!
Oh and btw: I got rid of Xfce too. I realized that all I needed anyway is a good WM (for which I use compiz fusion), an application starter (I still haven't figured out if I prefer dmenu or bashrun, so I'm using both), a freedesktop-compliant autostarter and that's basically it. Less is more!