Why? Xubuntu is a very polished distro and has gained many fans over the last couple of years as Ubuntu users dissatisfied with the Unity interface have looked for something more akin to the old Gnome 2 way of working, or wanted something that would run well on hardware no longer supported by Unity. The current version, 14.04 is an LTS release and is as good an Xubuntu release as I have used, and looks stunning - modern, but straight-forward.
Still, Xubuntu has always been a little 'heavy' for an XFCE-based distribution, both in terms of RAM usage and it's reliance on many gnome packages and dependencies.
Debian is a leaner system, though out-of-the-box, XFCE looks really dated on Debian.
The Goal: Create a pure Debian installation with XFCE, themed to look like Xubuntu, but with as few gnome dependencies as possible, whilst maintaining the same functionality.
How: Starting with a net install, install the packages one by one, check dependencies, replace with alternative packages if available, then configure and theme like Xubuntu.
Difficultly: intermediate. Some prior use of Linux is assumed, with some exposure to the command line and packaging tools, although all commands will be given in full.
Time: 1 -2 days. You will be installing a base system and then adding the packages one by one to build a complete system. You will be editing configuration files and using the command line.
This tutorial will be in a number of parts so that you can follow easily if you want to have a go too:
Part 1 - will get you a minimal Debian install with working X, sound, login manager and basic XFCE desktop environment.
Part 2 - will get the XFCE desktop themed like Xubuntu.
Part 3 - will complete the Xubuntu theming through boot up and log on.
Part 4 - will install all the required applications for a fully functional system and perform final tweaks.
Part 5 - will set up the system for remastering and create an installable live ISO.
Edit 20/06/2014 You can now download the entire tutorial in PDF format: