Slackware is the oldest of the current Linux distributions. There were a couple around before it, but they are long gone. So what is the secret of Slackware's longevity, the short answer has to be its simplicity. When many distros are adding more and more functionality with plenty of bells, whistles and eye candy, Slackware stays close to its roots. This means both its roots as Slackware but also staying close to Linux itself, with Slackware being considered one of the purest Linux distributions.
The twin goals of Slackware are ease of use and stability, but it achieves the former through simplicity rather than clever technical solutions, which goes a long way to helping with the other goal. The package management is a simple affair, although the package format has been improved for this release for greater efficiency, with far less of the dependency problems that afflict some other distros. The Linux kernel and other software included in Slackware are very close to the originals, Slackware introduces far less patches and modifications than other distros. This gave rise to a saying that is as true now as it ever way - "If you use Red Hat you learn about Red Hat, but if you use Slackware you learn about Linux".
This aspect of Slackware makes it a good choice for a general purpose distribution, it may not look as pretty in its initial defaults, but it is as capable ass any distro out there, and has a fiercely loyal, and helpful, community.
If you like your Linux distros to be simple, reliable and close to a Linux standard, Slackware is a good choice.