CentOS, Scientific Linux, FreeBSD, Windows Server 2008, Debian-Stable, Debian-Unstable, and (ugh) Ubuntu.

As runners-up to Slackware: Debian-Stable (c. 2007) and CentOS were quite good. I've heard worrying things about post-2008 Debian-Stable, but will take it with a grain of salt until I have a chance to work with it again.

Well, to be fair, the worst Slackware releases I can remember (8.1 and 9.1 both stress-tested and rejected with horror) were both based on the 2.4 kernel, which was having all matter of internal problems impacting system stability. The virtual memory and SMP mutex subsystems were being aggressively redeveloped, and like all new code, these redeveloped subsystems had a lot of bugs.

At the time I wished PV would just stick with 2.2 until the kernel developers sorted things out, but that probably wasn't feasible. Surely he did the best he could with such a crappy kernel.

Do you mean "push" in terms of how many packages your install, or how many daemons you run? I think you can run hundreds or more. The limit is probably the number of processes you can run, or on even larger machines, the number of TCP ports you can listen to (unless you learn the ways to have more than 65536 listeners).

Do you mean "push" in terms of how small a machine you can use? Pat gave a clue on that. One limitation is that newer kernels are bigger, as are newer libraries, and older packages often have security holes so you really might want to stay current (at 14.0 for now).

Do you mean "push" in terms of how many users you can support? I'd say Slackware is probably double what most other major distros can do, at least when comparing default setups of the others to an optimized Slackware (I find Slackware is more forgiving of optimizations I do).

I have setup many Slackware servers where I work. I've used 12.1, 13.37, and 14.0. I've also set up slackware on Intel, AMD and virtual VMware machines. I really have had no issues with Slackware. Other people have tried Ubuntu, Arch Linux, Debian and have ran into problems with them. I use Slackware for LAMP work.

FreeBSD 9.1 works really well for a system that Poeterring said was a joke and a toy. To me, he's the joke, and systemd is a toy, but enough talk about garbage... on to the real topic.

To be honest FreeBSD works very similar to Gentoo in design and setup with some subtle differences between them to emphasize that FreeBSD is not Linux. While I would not recommend you try FreeBSD without an Nvidia card as one is very much needed to get the most out of your graphics as Mesa3D still lacks a proper implementation for DRI acceleration with Gallium3D, FreeBSD is a worth the venture to actually learn as it's tools and setup are vastly different in many aspects, yet somewhat similar.

On a side note: I would also recommend you try out OpenIndiana as well as OpenIndiana (Illumos) is a true UNIX OS based on Solaris/OpenSolaris. It's a great project, so give them a good scrutinizing. You might like them.

LinuxFromScratch is a whole different beast of a Linux distribution.

I've really had to take it from a monolithic perspective, and reside myself into building the complete version locked system first from chroot before properly testing it. I'm aiming to get X11 and Xfce up and running soon so I can work within the main environment.

It is a lot like Slackware as you have to resolve dependencies yourself, but in all honesty, it's easy to do. The book lists the Recommended and Required dependencies, as well as Optional.

In ways it's also like Gentoo as the core LFS system is very minimal as well with you to build the rest.

To be perfectly honest, building LFS is addicting, and it's a great way to learn how your system works internally in ways even Slackware and Gentoo can't.

The one difference about LFS/BLFS is, truly, you have complete control of the system.

I'm already researching a way to graft Gentoo's OpenRC into the mix once I have the system complete to fully maximize my system. I love how OpenRC works and isn't just a replacement to SysVInit. It's basically SysVInit + Extensions (using dependencies and native systems to maximize functionality).

There are ways to even graft in Slackware's BSD+SysVInit system as well.