I am still a newbie myself, but I'm going to take a shot at answering this.....
Why Slackware? Lots of reasons! Some of the ones that attracted me are:
1. Stability Above all else, Slackware is built with stability and reliability in mind. While Slack isn't always bundled with the newest, shiniest releases of various softwares (or kernels; the current version of Slack (10.2) uses 2.4.x), but you can rest assured that every component of Slackware is tested, prove, and pretty much bomb-proof.
2. Flexibility You can use Slack for damned near anything from a simple email box to an enterprise server if you so desired.
3. "UNIX-likeness" It is generally held that Slack is the most UNIX-like of all the Linux distros. For some people this may be a minor or non-issue, but personally, this is a selling point.
4. Infinite customizability Slackware can be as simple or as complex as you'd like it to be.
5. Lightweight (if you want it to be). Related to the #4, Slackware can be tweaked to be the slimmest, trimmest, fastest distro you'll ever see. This is especially nice for less-powerful systems that would otherwise see more of their resources sucked up by more bloated distros (or *gag* Winblows).
6. It's *cough* l33t.
7. Knowledgable Community Having steeper learning curve than most other distros, Slackware tends to have some of the most technically savvy users in the Linux community as a while. If you're having a problem, it's pretty much guaranteed that you'll find a slacker who can not only help you fix your problem, but teach you every little what, why, and how about the issue that you've ever wanted to know. For somebody like myself who likes to know these little details, having such a group of fellows is most excellent.
There are other reasons, and I'm sure some of the others here can and will contribute, but these are some of the ones that sold me (ESPECIALLY number 5; I HATE bloated OSs/apps/etc, with superfluous crap that I don't want or need). I hope I haven't steered you wrong. To any veterans out there who may read this: please feel free to correct me if I misspeak, or add any additional information as needed.
Thanks, and welcome to the club(?).
-Surfrider