I have to disagree. Ubuntu is a great way to get to know Ubuntu. Debian, while _incredible_ has somewhat the same flaw, but in its case is worth the tradeoff.
Slackware or Arch are great ways to get to know Linux.
I must disagree with you in turn, in most particulars. Arch's wiki is an information source par excellence, and I find that the information contained therein is often just what I need to get my Debian system working correctly. The init systems will likely diverge at some point, but as far as I know that decision is not final yet, and I prefer systemd anyway. Are there some particulars you can divulge?
Ubuntu, like most mainstream distributions, has design choices and UI that abstracts away as much as possible the act of administering a Linux system. A lot of things you will do on Ubuntu are specific to Ubuntu systems, and even more so with distros like RHEL and Gentoo. Then you have oddities like GoboLinux.
It's one of the oldest sayings in the community that "if you know Slackware, you know Linux". This is somewhat less true for Arch but pretty close and still has the same spirit.
I've run Slackware, Debian, you name it and built my own LFS systems and run those as well. Slackware is as "raw Linux" as it gets, but due to frustrations with recent versions, I switched to Arch and haven't looked back for my Linux systems. It makes administration much simpler without having to do much that's specific to the distro and different from what you should expect in Linux.
That isn't something even remotely true about Ubuntu -- and apparently getting less so as time goes on.
tl;dr: what I'm trying to say here is that there are users and there are administrators. If you can't administrate your Linux system, you don't know Linux. Ubuntu tries to be a Linux system for users without needing to be a sysadmin. This is an idea that's "okay" I guess but in my opinion still not ready for primetime. Using a Ubuntu system isn't "knowing Linux".
While great points are made in each successive post, my child poster specified "Ubuntu with Gnome", which is a long time ago, in fact that was back when Ubuntu (IMO) was the best entry into Linux.
Now, I feel Ubuntu took away the administrative capabilities of a system that has not yet grown out of its need to be administered.
What stops you from saying that "Arch is a great way to know Arch" then? It's not as if it doesn't have its own package manager and all, like any other distro...
It's not the having a package manager that makes it specific, it's that it's ports-like. It's also a great feature that I'm a huge fan of.
It doesn't do nearly as much patching as other distros (save for Slackware or CRUX) to packages making reporting bugs upstream much more straightforward.