KDE is the most common and most popular desktop environment found on FOSS desktops today. It has the largest user share. It is the rare distribution that doesn't have KDE, and frequently it is the default choice.
KDE will feel right at home to the user coming from Windows. KDE is specifically designed to be user-friendly and consistent throughout. It is also the most widely developed-for desktop, and even a modest KDE install comes with enough programs to qualify as a whole operating system all by itself. KDE's programs are identifiable by the letter "k" preceding the program's name. All customization in KDE is done through GUI programs. The focus is very specifically on the non-geek user, especially one who favors the mouse and likes to change their desktop look every day. Discovering how to tweak it in geek-style from a text file is nearly impossible, as that's just not why KDE is here.
KDE is also the slowest FOSS desktop. While it has drawn much criticism in the past for it's pokey pace, it has perked up a little in recent years. A KDE program will occasionally crash, but since KDE is all things to all people all of the time, it's unavoidable that something will occasionally go haywire. I must say, once a KDE program has matured through a few versions, it never has a problem with stability again.
All of the KDE programs will run under any other environment with little fuss. While I myself shun the KDE environment, I have no problem adding "kicker", the KDE panel, to my menu in whatever environment I choose to run. There are some KDE programs I swear by no matter which desktop I run, such as K3B (the CD burner), KPOVmodeler (the modeling front-end to POVray), and some of it's games (kbackgammon, kolf, and kgoldrunner.).
I recommend KDE only for the non-power user, and with some reservation to newbies. While it is easy to use, it is vast as the tundra. The sheer number of choices available (six kinds of panels! dozens of configurations! combinations of theme, color scheme, and style to infinity!), and the vastness of it's features can take quite some time to absorb. KDE's help system, "khelpcenter", and it's one-stop all-purpose customization interface known as the "control center" are a lot of help. Getting to know KDE for the first time can feel a little like your first day exploring New York.
For developers, KDE has a ton of developer tools just like it has a ton of everything else. It uses the Qt library, courtesy of Trolltech. I can't tell you for sure whether Qt's really open-source or not - it sorta is and sorta isn't, and the controversy has raged like a firestorm. Qt has bindings for darn near every programming language in use today, but is native, of course, to C++. But what do you care what language - KDE has it's own IDE anyway! People who say Linux should be more like Windows need look no further than KDE to see most of their wish granted. But they say you should be careful what you wish for...