Similar to my Linux Distributions Described In Terms Of Beer posting a few months back, here's my thoughts on Linux desktops described in terms of beer. With my strong taste for German/Bavarian beers, most of the ones mentioned in this article are from the Austro-Bavarian region.
With GNOME's GBeers program sparking my interest in sharing my beer tastes as it relates to Linux desktops, I will begin with GNOME 3.x. I'd have to put Köstritzer Schwarzbier as a beer for GNOME3. Schwarzbier is "black beer" in German is a dark larger. The beer is strong and is not to everyone's taste, just as there's many people disappointed with the GNOME Shell and its fundamental differences compared to other desktops. There's also some stronger Schwarzbier from different breweries, which might be needed if you are one of the Linux desktop users opposed to the GNOME 3.x desktop.
For the vintage GNOME 2.x desktop, I would put it with any Märzen beer. This pale lager beer has been around for a long, long time just as GNOME2 was and it is extremely popular, especially at Oktoberfest. Some good Märzen beer includes any of the Munich Oktoberfestbier and Ayinger Festbier is also quite nice.
If KDE were a beer I would put it as any beer out of Weihenstephan. Weihenstephan claims to be the world's oldest brewery still in existence, just as KDE has been around longer than any of the other still widely-used Linux desktops. Sure, CDE (Common Desktop Environment) and others are older, but still not widely used. Xfce was also founded in 1996, the same year as KDE. Weihenstephaner tends to be quite tasty and their beers are very good.
Xfce and LXDE would go well as a radler beer like Stiegl Radler. Radlers are beer mixed with lemonade and common for German cyclists so that they're still able to cycle after drinking, hence its name. Radlers are lightweight but tasty, so they fit the lightweight Linux desktop environments well.
For Ubuntu's Unity desktop, I can't help but to compare it to a Bud Light or Miller Lite. Both Ubuntu -- and its default desktop -- are very popular just as is these beers within America. They're the most common beers in America and can basically be found anywhere. Many people like these beers, but there's also many that have a strong distaste -- just as there's a lot of people who aren't fond of Unity. Most Unity complains come down to not liking the design while lately there's also been many people disappointed in the desktop's performance. If wanting to compare Unity to a German beer, I would put it as Becks for the same reasons as the American beers.