Arch Linux is a distribution of the Linux operating system designed around giving users a lot of freedom to customize their system as they see fit.
If you go for Arch, this tends to be the main reason. I myself made the switch after growing increasingly dissatisfied with the course being plotted by the masterminds at Canonical towards a "friendly" operating system for people who don't really care about how computers work. If all you want is to check your email and play Farmville, this is not the distribution you are looking for.
Arch is a system for people who like control. Don't need window managers, wireless interfaces, or any of the other bazillion packages that come baked into most "modern" Linux distros? Arch doesn't shove them down your throat. If you want GNOME or KDE, they're right there in the package manager, but you get to make that call, not some developer in his apartment drawing names out of a hat at four in the morning.
Hand in hand with customization is a liberal dose of efficiency. Despite the fact that I do not even have solid state drives, my startup is still wicked fast and spends most of its time at BIOS checks rather than OS operations. This is something you will notice across the board with many Linux distributions if you switch from Windows or Mac OS, but Arch has in my experience had a performance edge on most others.
Another speed boost comes from the option (should you take it) to run a lightweight desktop environment. I use LXDE and OpenBox, and the responsiveness is phenomenal. Compiz and other fun things are naturally supported too, but the OS doesn't try to default you into snazzy, slow animations and compositing like some other distributions have begun to.
One of the features that separates Arch from other Linux distributions is its lack of a release cycle. Updates to various packages you have installed are simply made available as they are developed, which means an Arch user tends to be running fairly close to the latest and greatest of any application he chooses to install. Detractors (frequently of the Debian variety) tend to make noise about how this makes the system very unstable and you could crash and burn without warning. This has not been my experience with Arch at all, but your mileage may vary.
The great attractor of the rolling release is, of course, that you don't have to wait an eternity every 6 or 12 months to redownload the entire OS when the developers decide to bump the version forward. Particularly if your Internet speed is slower than you'd like, this can be a huge benefit.
Another great selling point of the Arch community is its user repository, a database of source packages that have been unofficially used in Arch. While official repositories are quite timely and cover almost everything you might need, occasionally you need something far from the beaten path. In many distributions this requires collecting, building, and manually installing source from the application developer, which mucks around with your carefully constructed package management setup. Arch, on the other hand, provides a friendly interface to build custom installer packages from developer source which are recorded by pacman and play nice with dependency-management and all the other goodies we have come to expect.
Similarly, Arch supports the fantastic Arch Build System, which allows users to locally compile any package available in the official repositories for all the goodness that provides.
The Arch community, from my experience, is quite top-notch. Vital news about architectural shifts is posted to the front page, helpful when pacman has a complaint about overwriting critical configuration files. Their community forums and IRC channels tend to be responsive, if a little overzealous to RTFM.
Overall, I recommend strongly Arch Linux to anyone who is looking to make the jump to a more flexible, bare-bones Linux distro but isn't quite ready for the madness that is getting Gentoo up and running.