I understand why Ubuntu doesn't switch the default to KDE; there's too much history and people would recoil at such a big change. That said, I always run Kubuntu rather than the default Ubuntu desktop and I'm really happy with it. Switching to KDE is almost as smooth as it can possibly be: once I have Ubuntu installed, all I have to do is "sudo apt-get install kubuntu-desktop" and then choose KDE at login.
Some of the reasons I really like KDE:
* I have keyboard shortcuts for switching desktops; I feel much more productive with 9 easily accessible virtual desktops. I use the "windows" key, sometimes called "meta": meta-Q, meta-W, meta-E, meta-A, etc.
* I've added rules for automatically changing the window title bar colors to make it easier to distinguish between very similar-looking windows. My Sublime Text windows in particular have a different bar color for each project. Sometimes I have as many as 4 projects open at once.
* I can lay out and enhance my screen any way I like. KDE doesn't quibble and even encourages it.