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Arch Linux keeps things simple - it's lightweight, without the cruft of most other distributions, and completely flexible. These traits make it a popular distribution with many more experienced Linux users.
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Pros
Pro Helpful community
If searching through the wiki or the forums for any problems turns nothing, any question on the official forums, Arch subreddit or the IRC channel will be answered within minutes. There will probably be no hand-holding however, Arch users prefer to point anyone to a resource that may help them instead of trying to outright solve their problem in a forum thread. This is quite helpful for people who want to really learn how their system works but also for other people who may stumble in that thread considering how most problems don’t have a universal solution.
Pro Excellent package management with the AUR
The AUR is a repository with a very extensive catalogue of build/install scripts that are contributed by users. While these scripts are inherently less secure than conventional packages maintained by a distro's authors, it's still way easier to verify the security of install scripts than it would be to write them yourself. It's very usefull.
Pro Excellent documentation
Arch's goal of simplicity means there's usually one preferred way to get things done - through organized and well documented configuration files. This focus, combined with the community's recognition that configuration files can be intimidating, has resulted in excellent documentation that's accessible to newcomers, and very instructive about how Linux actually works. The documentation is often so thorough that, when searching for solutions to problems while using other distributions, such as with video card drivers, oftentimes you'll find the most effective solution in the Arch Linux wiki or on the forums.