The biggest draw of Arch for me is pacman. The system itself is incredibly simple (and incredibly fast), but more importantly, it's really easy to design and compile your own packages for it. To make a Debian package, you have to edit a bunch of control files, run a bunch of shell scripts, and then it packs all that up in some obscure cpio-based format.

With Arch, on the other hand, it's one program - makepkg - with only one or two "control files" (PKGBUILD and maybe an install script), and it generates a nice, easily-inspectable .pkg.tar.gz file. Plus you can find just about any package you could want in the AUR, and if not, you can find one that builds like it and then modify it to work with your package.