I have to recommend Gentoo for anyone who doesn't mind learning internal workings. Binary OS have too many problems with application availability, and while Gentoo adds some new scary issues (such as forseeing what programs you want to install a few hours in advance), it's at least a step in the right direction.
I second the Gentoo motion. Gentoo teaches you much.
For years my fascination with Linux was largely that I was attracted to the Philosophy driving it, and as I sincerely had a hard time reading man pages (call it a form of dyslexia), I could never run a linux box without help.
Until I met Gentoo. Doing the simplest upgrade of Apache+PHP, I spent hours HOURS learning about the emerge system because PHP is considered experimental and recent releases are all kinds of INTENTIONALLY complicated, in order to keep newbies from breaking their system by installing PHP. Well I needed the most recent PHP because of object-oriented and PDO improvements.
Anyway, long story short: A few painful days with Gentoo taught me more about Linux than years of tinkering. It was painful, painful indeed, but now I have a sense of what's happening under the hood with Linux, and that's worth its weight in gold.
Now that I "get" it, Gentoo is a breeze, and if Gentoo is a breeze, what's next? Slackware? That used to be the 'hard' one. Is it still difficult?
Course, all of this is because I don't like Kubuntu politics, and thus I'm still on topic.
-Jared