I've tried for years to like KDE, and I just can't. It's too *busy*. It's the first desktop I've ever sat down at that I couldn't just use right away - I clicked on a button, and up popped "Activities". Creating a new activity left me with a blank screen and nothing to do. Everything is animated and glowing, with huge distracting icons and drop-shadows.
GNOME is all right. GNOME 3 might be weird, but at least it's trying to do something other than emulate Windows or Mac OS X. It's just too buggy for my tastes.
XFCE is all right too, but I was turned off by how haphazard and...unprofessional Xubuntu was. I didn't like having to explain to my eight year old nephew's mother why he was asking what "Gigolo" did, for example.
Unity, despite its many faults, comes with Ubuntu. Despite *its* many faults, Ubuntu is the only open source OS I've used that actually seems like an integrated product. With Unity on Ubuntu, you don't get things like "Gigolo" which is just stupid or "lxrandr" which is inscruitable. You don't get a million different ways to customize things down to where you can make your desktop look like an angry fruit salad. That may or may not be a good thing.
Also, say what you will about Mir but Ubuntu is at least trying to make an integrated system. The other desktops are really poorly integrated with the rest of the system, resulting in my having to explain to my father "No, you're using Debian" "I thought I was using Linux" "You are, it's the Debian distribution" "Why is this called GNOME Terminal then?" "That's the desktop environment" "This says I'm using X windows" "That's the underlying display architecture..." Users of Windows don't know what GDI is unless they're looking for it. Same with Quartz and Mac OS X.
I hate to say it, but the non-baseline-Ubuntu distributions are not really doing a great job of making a desktop operating system. Like was said the recent thread on Fedora Core's newly-proposed model: they're just a bunch of products from different people thrown together into one mass. I appreciate the amount of effort the distributors go to, but Ubuntu has gone just a little bit farther and made something that feels like a modern, unified operating system. Some people don't like that, but a lot do.