Recs.
Updated
Xfce is a lightweight desktop environment for UNIX-like operating systems. It aims to be fast and low on system resources, while still being visually appealing and user friendly.
SpecsUpdate
Pros
Pro Classic and Familiar
Xfce works very much like the classic Gnome & Windows desktops, taskbars (panels) and desktop icons, letting you get your work done without being frustrated.
Xfce embodies the traditional UNIX philosophy of modularity and re-usability. It consists of a number of components that provide the full functionality one can expect of a modern desktop environment. They are packaged separately and you can pick among the available packages to create the optimal personal working environment.
Pro Adheres to standards
A priority of Xfce is adherence to standards, specifically those defined at freedesktop.org allowing for interoperability and shared technology for X Window System desktops. This interoperability is particularly significant for users looking to, e.g., run alternative window managers.
Pro If you used to love old Gnome DE you will love Xfce too
For people who were dissapointed with Gnome 3, Xfce can be easily customized to look like Gnome 2.x
Pro A true UNIX Desktop Environment
Xfce adheres to the UNIX philosophy, which means it strives for being modular, minimal and expandable. This makes it very much customizable. You can make it as minimal as you want and as heavyweight as you want depending on the features and modules/plugins you use.
Pro Can mimic HiDPI supoprt
This requires adjusting settings. In Settings Manager → Settings Editor → xsettings → xft DPI can be adjusted. Setting it to around 180 should work well for "Retina" screens. While not full HiDPI support, this can help mitigate small text on a small HiDPI screen.
Pro Window manager (XFWM) is a compositing WM by default
By having a compositing WM as the default WM makes way for a lot of visual tweaks and tricks that can and do make Xfce look great. You can adjust the transparency, shadows, borders, etc. and many other advanced tweaks are also available.
Cons
Con Very slow development
XFCE tends to be extremely slow in it's development. The last release was Xfce 4.12 on Febuary 28, 2015, 3 years ago, with the next release still remaining unknown.
Con Missing some basic functionality for a desktop environment
Xfce is missing essential functionality like a file-archiver or a polkit-client, so you have to find alternatives for those applications (eg: by stealing them from MATE or GNOME, however this adds additional dependencies that will bloat Xfce).
Recommendations
Comments
Flagged Pros + Cons
Con Default is a bit Ugly
The default setup of the UI is a bit bland and looks dated, very similar to Windows XP or earlier. While it can be customized to look more modern, what is presented upon first install of the DE is not the prettiest DE out there.
Pro Classic and familiar
Xfce works very much like the classic Gnome & Windows desktops, taskbars (panels) and desktop icons, letting you get your work done without being frustrated.
Xfce embodies the traditional UNIX philosophy of modularity and re-usability. It consists of a number of components that provide the full functionality one can expect of a modern desktop environment. They are packaged separately and you can pick among the available packages to create the optimal personal working environment.
Pro Highly customizable
Xfce offers plenty of settings, and even things like theming XFWM is a simple task (it's just a handful of images.)
Many possible permutations of window colors, borders, fonts, etc. Compositing can make it look downright sexy.