1. The speed
XFce-4 is as fast as any Window Manager/Desktop Environment on my Pentium4 2Ghz, 256MB of RAM, and Geforce4MX. I recently compared it to fluxbox and Windowmaker, some of my other favorites, and there was basically no increase in speed or reduction of memory usage. The entire Environment feels more responsive.
While this may not be the most feature-packed Window Manager, it has some things you'll appreciate. When working with GUIs, I am more of a mouse man. The main thing I do with a GUI is browse the web, so not having to take my hand off the mouse helps me a lot. One thing I like about XFWM is that you can use the scroll wheel to shade a window. All you have to do is scroll over the title bar. Very useful for quickly seeing something behind a window.
XFce-4 also includes a large number of Window Manager themes. Some are ports, and some are originals. There should be a perfect theme for anyone. Personally, I like the "Bluecurve" clone. It has other standard features. You can choose the location of the buttons on many of the themes. This is useful for people who are use to non-standard button locations.
One annoyance in XFWM is that there is no hover effect on the buttons. Most other Window Managers can do this, and it helps you decipher which button you are currently hovering over before you click it. This would help to prevent accidental closes.
3. The Panel
One thing you may or may not like is the lack of a "Start Menu". This is standard for KDE and GNOME, but for XFCE, they use a CDE-like Panel. I've fallen in love with the panel. Its a great all-in-one utility. You have "shortcuts" in fully organized sections, you have Drag-N-Drop between the File Manager and Panel, and you have a myriad of plugins for many different uses. The plugins really make the panel nice. I like the mini-commandline plugin, the notes (stickies-like) plugin, and all the system monitor plugins.
This is the Root Window utility. It's rather standard, but has some advantages. Using the Middle Mouse button will bring up a Virtual Desktop menu, and using the Right Mouse button will bring up a "root menu".
One problem with XFDESKTOP is that there is no GUI for editing the menu. The menu format is not too complicated, but it would be much better with a GUI. One program that I love to use in combination with XFDESKTOP is "Menu Maker". It is a python-driven automatic menu generator. It works with Fluxbox, WindowMaker, XFce-4, and others. With one command, it will automatically generate and install a root menu that contains all the programs you have installed. It has the ability to recognize a large number of programs, so it does a lot of the work for you.
My favorite feature of XFCE and specifically XFDESKTOP is virtual desktop wheel scrolling. If you use your scroll wheel on the root window, it will allow you to scroll through your virtual desktops. This is something I can not live without anymore. I usually have 2-3 Virtual Desktops with applications opened on every one,and it lets me switch through applications faster than any other method I've used. This includes minimizing/maximizing, Expose, and "Sloppy" focusing.
Also, XFDESKTOP does not have the ability for desktop icons. Some people will probably care, but I don't. The panel allows me to stay very organized.
This is the XF File Manager. It has a lot of nice features, though maybe not very original ones. It has bookmarks, a visual Diff, an FSTAB explorer that allows for easy disk mounting, an integrated samba viewer, a "Trash Can" like feature, and a visual file finder. All these parts work very well.
It will also remember your application preferences for file extensions. It doesn't seem to have too many programmed in by default though. In Environments like KDE and GNOME, almost any file you click on will be opened up in the proper viewer.
The File Manager is different than Konqueror/Nautilus in that it uses a tree-view. As you can see in the screenshots, it's something you may not be adjusted to. Personally, I prefer the Nautilus/Konqueror way of showing a group of icons, with no tree like structure by default. I think the main reason is that when opening a new folder in XFFM, it's slow to display the contents. I have a large folder of images, and it loads in half the time on Nautilus.
But, like the rest of XFCE, the File Manager is optional. XFce-4 is very modular, and follows freedesktop.org standards, so it stays inter-operable with other environments.
XFce-4 uses GTK2. This, along with a number of Window Manager decorations and it's own GTK engines help to create a nice and smooth look. XFce-4 is a very clean environment. I personally prefer GTK2 over QT (looks wise), so its a nice fit. One feature I liked about the XFCE GTK engine is that it puts a thin black border around selected boxes (similar to OSX). This prevents accidentally typing into the wrong field.
XFce-4 may not be loaded with eye candy, but it supports all the modern features you expect, like Anti-Aliased fonts and complete themability.
XFce-4 has the best Xinerama support out of any Desktop Environment or Window Manager I've used. Windows always show up on the proper monitor, maximization is always perfect, and simply put, I forget that I am using two monitors. This never happens on other Desktop Environments. I'm always having an issue with the wallpaper spanning, or windows showing up on the wrong monitor, or some other equally annoying thing in KDE and GNOME.
Most of the other features of XFce-4 are standard. Keybindings, GUI Preferences, Mouse preferences, run box, and taskbar are not exceptional. Now, you may be wondering if that is it. Yes, that's basically all XFce-4 contains. It is VERY lightweight in comparison to KDE and GNOME. I consider this it's advantage. Startup is less than 2 seconds on my main computer, and 4-5 seconds on my 350Mhz K6-2. Along with that, XFce-4's memory usage is very slim. The complete environment only takes up about 30MB on my computer. Whereas GNOME or KDE would take up 100-150MB if I loaded everything. I have my application preferences. I use the console for most things, but I do have GUI preferences, and XFCE is sure not to "choose" for me. I don't need most of the applications that KDE and GNOME provide. It's that simple. Choose XFce-4 if you already have a group of programs you prefer, and just want something to manage your windows, shortcuts, etc.
XFce-4 should definitely be considered one of the big 3 Desktop Environments. It manages to have a perfect mixture of speed and features.
Also, you may note lack of a Session Manager. There is currently one in the XFCE CVS. I have used it and it seems stable, so if you require a Session Manager, you can try it out.
Menu Maker: http://www.menumaker.sourceforge.net
XFce-4 Panel Plugins: https://developer.berlios.de/projects/xfce-goodies/