For me, the biggest point in favor of Arch is that it is a binary distro that seems to be closest to "vanilla" - meaning that packages are generally built directly from upstream, without heavy distro-specific patchsets applied by maintainers. There's still plenty of patches, mind you, but they're usually there to ensure that it builds and installs correctly in Arch environment, not to change functionality.

I also find Pacman to be noticeably faster than all other binary package managers. I just did a full package upgrade on a laptop that hasn't been upgraded since the beginning of this year, and yet it was able to compute a (fairly lengthy) list of packages to upgrade faster than I could blink.