Slackware is pretty close to "bare metal" in terms of your options for easing system administration. Every Slackware network ends up being its own unique animal, as most of the automated system administration tasks need to be cobbled together by the sysadmin(s), since there are not the pleasantly slick tools you'd be used to from other distributions. Software management is a blast from the past with Slackware: yes, it has what they laughably call a "package manager" for Slackware, but it's pretty primitive and rudimentary beside tools like APT and YUM.
Gentoo has excellent software management tools, and great documentation on the Web. Unfortunately, it's also the "What's borken this week?" distribution. It is very much a bleeding-edge-oriented distribution, and ports tend to get included before adequate testing has been done, which means that updating software versions is really an adventure with Gentoo. This can be something of a challenge for production systems, where uptime and stable functioning are of primary importance.
NetBSD is kind of in a precarious state right now. The community is getting shaken up by internal turmoil, it has been losing popularity against FreeBSD and OpenBSD for years, and there are some serious questions about the effectiveness of the core team. The major claim to a benefit of using NetBSD is portability: there was a time when it was the most-ported general purpose modern OS in existence. Debian has surpassed it now, however, not only because Debian has been ported to a myriad of different platforms but also because NetBSD has been slacking on the platform porting. Its portability is now, mostly, a thing of myth and legend, from what I've seen.
OpenBSD is widely known as the most secure modern, general-purpose, easily available OS in existence. The OpenBSD project, in fact, was the source of the OpenSSH implementation of SSH, which is the most widely used secure/encrypted remote administration tool in the world. All major Linux distributions include OpenSSH by default, along with a number of other OpenBSD-originated tools. OpenBSD has had a grand total of one remotely exploitable vulnerability in default configuration, ever, in the history of the project -- and no documented security compromises in default configuration at all. It's about as stable as an OS can possibly be while still allowing you to do things like run a GUI web browser such as Firefox. On the other hand, it doesn't benchmark very well on system performance for a number of common tasks, in part because of all the internal security auditing it does. Additionally, in part because of the extremely close attention paid to security and stability, OpenBSD has very limited hardware support capabilities. Don't expect to play Unreal Tournament on OpenBSD any time soon.
While the size of package repositories is not necessarily of primary importance in an OS, there are other reasons to consider the availability of software in the software management system of choice for your OS. For instance, both Gentoo and FreeBSD use a "ports tree" to make commonly used software available to you in source code form with great ease. You can install the software from source in the ports tree, without having to use a package manager. You can also use a ports manager, such as emerge for Gentoo or portsnap/portupgrade for FreeBSD, to make the process of handling installation from source as easy as installing binary packages is in certain Linux distributions (such as using APT with Debian).
Because it's installing from source, however, you have greater control over the configuration of your software at compile time, you know exactly what you're getting whenever you install something, and stability of both the operating environment and the software management system is possible. The Gentoo project plays fast and loose with its ports, so this benefit is mitigated somewhat in Gentoo, but the FreeBSD project tends to lavish a fair bit more diligence on maintaining the stability of the software in its ports tree. In addition, because you're installing from source, there are fewer detriments to going "outside" the software management system of the OS to install something than there are with a system like APT or YUM.
There are benefits to using a system like FreeBSD's ports tree over just downloading tarballs from Sourceforge, as well. For one thing, it's a lot easier to search through available software to see what you want, since there's searching functionality built into the ports system. For another, the software is all vetted and extensively tested before inclusion in the official ports tree, and modified for maximum stability on FreeBSD. Software in the ports tree is by default configured to share libraries so that you don't need eight different versions of a given library to support eight different applications you've installed from tarballs -- or, worse yet, so you don't have two applications installed from tarballs that share a library, then one of them on a version upgrade changes its version requirements on that library dependency, leaving you with a system where one or the other application is broken unless you reinstall them both. You also get the benefit of increased security, because there's functionality in FreeBSD's software management system for checking for new security vulnerability patches and automatically auditing the entire ports tree, so you don't have to track vulnerability announcements for each and every separate program you've installed from a tarball you downloaded from Sourceforge.
Despite all this, you don't even have to install from source in FreeBSD if you don't want to. There are binary packages available for when you don't want to deal with compilation and are happy to go with all defaults. The packages, however, are basically exactly the same as ports, except that the process of compilation is already done for you -- so the whole system, between packages and ports, is unified. In fact, when you want to uninstall a port, you can use a package management tool to do so. Thus, once installed, packages and ports are interchangeable and indistinguishable, with the exception of the fact that if you installed from a port the system will remember to upgrade by compiling from the new version port rather than installing from the new version package. It's really a beauty to behold how well the system works as an integrated whole.
Oh, yeah, and audio configuration for FreeBSD is a lot easier in most cases than for Linux. That was a very pleasant surprise for me.