Just a couple minor things

* 'Linux' isn't an acronym; typically people just spell it 'Linux'. Some people like to say "GNU/Linux", since technically 'Linux' is just the kernel. 'UNIX', however, does need to be capitalized.
* We usually call different 'versions' of Linux as 'distributions' or 'distros'.
Some longer things:
* Ubuntu is based on Debian. Although Debian, Arch, etc use a lot of the same software, there are some fundamental design differences. Most of these are pretty low-level though. Both Debian and Arch are considered to be fairly minimalist (especially the latter). Ubuntu, which is based on Debian's packages, is also designed quite a bit differently from it, and comes pre-installed with a lot of software, aiming to be "user-friendly". In fact, Ubuntu is so different from Debian nowadays that several Ubuntu packages won't work on Debian anymore, even though they both use .deb. Mint is basically an even more user-friendly Ubuntu (it also comes in a Debian flavor). Fedora and CentOS are basically free versions of Red Hat Enterprise Linux. OpenSUSE was originally Slackware based, but is now its own standalone thing. Gentoo is mostly insane, but I respect it; users must compile all software from source code on it.
* Arch Linux has comparatively fewer users than most of the others (it's steadily increasing though!). However, since there are no known reports of problems from other distros, we can't assume it to be a problem resulting just from Arch not being Debian-based. Fedora, OpenSUSE, etc all use repackaged versions of Steam as well. We also don't even know if this issue affects all Arch users.
* CK2 runs flawlessly on Arch, with the exception of major DLC. The game works ~ the DLC mechanism is just a little bugged. It's a great game, too.
The DLC's apparently may have worked on Arch in the past, so it may be a regression in a newer version. One person (Agustina) reported version ~1.10 as having worked for them. I say "may have worked" because we don't know what distro that person was using. I'm going to see if I can get a copy of that version from Youcke to test it out.
All of my other Steam games run flawlessly on Arch, so I'm sure there's a solution around here somewhere. Indeed, as I mentioned, CK2 runs flawlessly apart from major DLC's. Heck, I can even run a lot of Windows software in Arch without problems.