I know this is a pretty generic question and might be slightly difficult to answer, but is there a particular Linux distro which requires a lot of learning to use? I want to get a little more involved with the way Linux works and that's not really something which Ubuntu helps with (damn user friendly shit).
So, throw some very user un-friendly distro's this way
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Here is a link that might help you out.
http://www.tuxradar.com/content/how-choose-best-linux-distro
Also, if you're up for it: http://www.linuxfromscratch.org/
Each major distro has it's own way of doing things. Arch Linux in particular keeps things simple with single init scripts for daemons that start up at boot instead of linking several daemons, from several different directories for different run levels. I like to keep things simple so I use Arch Linux but because of that I do things the 'Arch Way', not the 'Debian Way' or the 'Red Hat Way'.
On top of that, you can still get your teeth stuck in to something like Ubuntu. Just because it's got a GUI doesn't mean it's for noobs.
I highly recommend Arch Linux if your after keeping things simple, and you will definitely learn things, but everything you learn, you can probably do on Debian and it's derivatives.
Having said that no other distro, I feel, has as good a documentation. Just google "arch linux <something>" and you'll come across the wiki article.
The only problem is the lack of package signing because the pacman dev's can't be bothered. Don't let that put you off though, dive right in:
https://wiki.archlinux.org/index.php/Beginners'_Guide
Also, force yourself to change, use vim instead of nano and most importantly, your computer is your bitch. Not the other way around.
I would not advise you to go Linux from Scratch until you feel comfortable in a command line and know what you like and dislike in a distro.
Damn, now I'm tempted to install Gentoo again.