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Slackware attempts to be BSD-like. As such, if you prefer the Slackware way of doing things over other Linux distributions, you might prefer just to use BSD altogether. FreeBSD offers many of the features you mentioned you like, as well as those you feel Slackware should have but doesn't (e.g. documentation, ports/packages).
I know when I first tried out open source operating systems back in 2000, I ended up trying out in order (just by installing and then getting NAT to work), Redhat 6.0, SuSe ?, Slackware 7?, and FreeBSD 4.1. I stuck with Redhat at first, simply due to its popularity. However, after a while I decided that I wanted to learn unix and not Redhat's user tools. Since this also ruled out SuSe, it became a tossup between Slackware and FreeBSD. Simply due to documentation (the FreeBSD Handbook was and is excellent), I went with FreeBSD.
Often, I hear people make arguments against the BSD's since they are supposedly "more difficult" to use. I guess that depends on what you deem difficult. Although I was doing things by hand in FreeBSD, it was straightforward enough as to what was going on, as opposed to netconf or linuxconf where I was never quite sure what was going on. In particular, I found getting NAT and compiling the kernel much easier. One thing I've always hated about compiling linux kernels is you end up either with menuconfig, where you have to scroll through a gazillion irrelevant options to get everything you want, or edit by hand with zero documentation. Copying the GENERIC kernel config file and copying a few well documented lines over from LINT on the other hand was pretty painless, especially with copying and pasting with moused in console.
Over the years, I've run other linux distributions as well as given the above ones additional tries. I currently run Debian Potato as one HomePNA2 (phoneline) to ethernet bridge, and Slackware 8.1 as the other bridge, as the closed source HPNA2 nic's don't have BSD drivers. I also run Solaris, Irix, OpenBSD, and NetBSD.
To this day, I find FreeBSD the easiest to use; due to consistencies in the design rather than point and click gui tools. I still find FreeBSD's package management system to be the best due to having both source and binaries interoperable with each other. When I tried Gentoo, the kernel crashed as I was installing ALSA drivers so I deemed it too unreliable. This was 2 years ago though to be fair. I hear it has improved considerably but can still be testy at times. However, stability aside, I just don't find it quite as cleanly setup as the BSD ports system it was based on. I just can't imagine anything easier than browsing a directory on my filesystem for all the apps I need, and using the "make" command in the right subdirectory to install what I want, just as if I was doing it directly from source only having all the dependencies and package management worked out for me automatically. pkg_install <pkg> is also easy for binaries (just like on Slackware).
Another argument I often hear made against BSD is hardware compatibility (or lack of it). Aside from those HPNA2 network cards, everything I have works fine in FreeBSD. Nvidia has offered graphics drivers for quite a while now, which you can simply install through ports. All the popular network card chipsets are well supported, even onboard nforce2, gigabit and wireless. In the early days of wireless, the wi driver in BSD worked much better than the linux-wlan, allowing configuration with just ifconfig, and even had the feature to run in ap mode much earlier than linux did. My soundblaster live card has always worked well, needing only a single line in my kernel config to work (for all pci based soundcards). Even my ATI TV Wonder TV card works perfectly.
Anyway, for anyone who likes Slackware and hasn't tried FreeBSD yet, I highly recommend it. For any technically minded person it is easy enough (sometimes easier) to learn, and also offers a lot of other nice features.