Linux

For me, it's:

• All software is shipped with as few changes as possible from upstream, so I'm getting the software as intended. If there's an issue, it's likely due to the software, not my distribution's unicorn configuration.
• Pacman. This includes PKGBUILDs, syntax, and speed.
• Good support. For all that this distribution isn't "the standard", you find install instructions in places you wouldn't expect, and more difficult things tend to work on Arch more easily than on other distributions.
• Easy to set new things up. Because Arch doesn't ship with much configuration, there's no existing configuration you need to investigate in order to wrangle it to work with something new. This is also a downside, but we'll get to that...
• Inertia. I installed it a few years ago, and I kind of want to move to openSUSE or Fedora, but I'm too comfortable here.

Downsides:

• You need to configure everything. That includes the security stuff like AppArmor and SELinux you don't understand.
• Occasional breakages. Arch doesn't break that often, but it's annoying when it does. Usually visiting bbs.archlinux.org is enough to set you on the right path.
• Some software is packaged more slowly than other rolling distributions. Notably, GNOME is usually packaged a few months after openSUSE and Fedora ship it.
• Constant updates! And HUGE updates, at that! Not great for computers you don't use often. If you do, make sure to pacman -Sy archlinux-keyring before you install new updates.