Arch takes a few decisions I don't like. The rolling release bring a big one, and it is somewhat antithetical to Slackware's approach of a release every few years when Pat feels like it. The other one is systemd -- Arch was one of the earliest adopters. Slackware has long prided itself on being Unix-like, and nothing about systemd's design seems in line with the Unix philosophy. They do attract a similar class of power user though, as do Gentoo and NixOS. Particularly those not scared to get their hands dirty.

If I recall correctly though Arch is pretty good about avoiding making things unduly overly configured, preferring a mostly vanilla install of software where possible. This is SO crucial and is probably one of the biggest places I lose my mind with Debian derivatives. I shouldn't need documentation for using a package on Debian; reading software's official documentation should be sufficient to be accurate without major caveats about what was modified.