There are a few reasons I have mentioned in the past as to why I prefer Slackware to other Linux solutions:
1. No patches on packages (unless absolutely necessary).
2. No unstable packages used, not even in the -Current tree (generally speaking).
3. No dependancy check package manager (and that's a good thing 'cause the third party packagers have some common sense regarding their releases).
4. Simplicity of design, configuration and usage.
5. Because of #1 and #2 performance is pretty good (comparatively) and stability is rated great.
I am running Slackware-Current for months now and whenever are updates available, I use the swaret tool to upgrade the OS to the latest "unstable" version. Funnily, this "unstable" version is much more stable than any other popular distro I test every now and then for OSNews. Fedora is called a "beta" but it is plugged with gazillion bugs (I think I filed about 7-8 bug reports so far for Test2) and it is incredibly slow comparatively, Mandrake Linux 10 Community had its share of bugs as well as I noted recently on my review. The fact that no patching is taking place, in general on Slackware, helps the overall stability and performance, because the developers don't have to deal with new difficult-to-discover bugs because of it. If a package works out of the box, then it means it is approved to be included on Slackware, otherwise Slackware will either keep an older version around or dismiss the package from their distro. This might seem to some readers that Slackware uses old packages but this is not true at all. Anything that is declared "stable" by its developer and it does compile/run without additional patches is good to be used on -Current for further testing.
So, in the latest -Current tree you will find KDE 3.2.1, Gnome 2.4.2, kernel 2.4.25, Gstreamer 0.6.4, latest ALSA, latest Gaim, XFree86 4.4.0, Qt 3.3.1, Gtk 2.4.0, XMMS 1.2.10, autoconf-2.59, automake-1.8.2, gcc 3.3.3 and more. Using the swaret tool it makes upgrading a real breeze. You can also download third party binary packages from LinuxPackages.net.
As you saw above, the distro comes now with XFree86 4.4.0 and as the founder told us "we will be the last people switching to something else". In other words, if it ain't broken don't fix it.
Slackware offers kernel 2.6.5 as a source download on their servers under the /testing tree, but it doesn't seem probable that a 2.6.x kernel will be trouble-free enough for Slackware's standards for the next Slackware release (however it might offered as an optional download on /extra). The real questionmark though is if Slackware will upgrade in time for Gnome 2.6.x. In any case, the Dropline Gnome distribution will have Gnome 2.6 packages out soon. Rumor has it that next official version of Slackware will happen at the end of April.
Remember though, Slackware is not for everyone. If you just want a home system with GUI configuration tools, you better look elsewhere. Slackware does require some initial tinkering and it doesn't have gui tools for most things. However, its magic is that after you have managed to setup the system the way you need it to, it works wonderfully and without problems.
As with all products though, I do have my own requests. My biggest problem these days seems to be the fact that the Slackware online book hasn't being updated for ages. There is literally no up-to-date information in one place anymore for Slackware. And the online book is lacking severely on how to configure services (while this is an easy process most of the time, new users do require this information in the book). Other than that, the system seems to be rock solid, compatible and generally: sane. Please read here for more of my opinions on Slackware and many more screenshots.