It's true that it might seem like an anachronistic distribution in 2012. It was actually the first distro I ever used back in the day when I started running Linux full time and increasingly had to interact with it professionally, and while I've long since moved on to other distros for a myriad of reasons, I wouldn't trade the years I spent running Slack for anything in the world,for a very simple reason, and that is that the old adage is absolutely true: when you run Distro X, you learn Distro X. When you run Slackware you learn Linux.
The whole distro, from the package manager to the init scripts is built around the KISS principle. Take the init scripts for example, they're nowhere near as powerful as, say, Upstart, but they're orders of magnitude easier to troubleshoot, and if you take the time to read through them (and yes, they're actually readable and very well commented) you gain a great working knowledge of how Linux actually boots and gets everything up and running.
The same philosophy applies to every other corner of the distro. Want to learn how to compile stuff on Linux? Take a look at a Slackbuild script, everything is in there. Want to know exactly what goes on during a distro upgrade, like what packages have precendence and so on? Read UPGRADE.TXT (http://ftp.slackware.com/pub/slackware/slackware-current/UPG...). I can only dream that even a small percentage of so-called enterprise software I've come across had release notes and upgrade guides this clear and well written.
Another great thing is that they maintain a policy of minimal interference with upstream code. If you come across a bug, you can be fairly sure it's from upstream and not Slackware, instead of having to examine the gazillion patches other distros like Debian and its derivatives introduce into the package.
I could continue to wax poetic about it, but I think I've given you an idea. It's not an exaggeration to say that I probably wouldn't be working in IT today if I hadn't come across Slackware in the before time. It's fun, and if nothing else it's a great learning experience.