I've never really understood this kind of question... what does a person mean by stable?
From a default install, Slackware vs. Debian vs.... almost anything, they all should be pretty comparable. Once a person starts adding destabilizers, i.e. third party packages, mixed repositories, random slack builds from untrusted sources and unknown build environments.... things could get dicey, again, for just about anything. I wasn't going to reply to this thread at first because it looked like a zombie, but it seems to be actively breathing again, so I thought I'd throw in my two cents (Canadian currency, so take that for what its worth ;) )

Stability is a function of the administrator, not the OS in my opinion. Distribution maintainers typically put out stable, functional OS's that are tested as much as they can. They try to ensure that what they are offering will work /as released/ for their target audience.
The only OS I've had that really got unstable was Fedora Core 4, when I was first getting interested in Linux/GNU based OS's. The only reason it became unstable for me was overzealous package addition and not knowing about the dangers of mixed sources. My perspective is from that of a desktop, hobbyist user. The above example of server clusters and the like is beyond me!

That thought crossed my mind when I attempted to migrate our schools' network (two servers, twenty desktop clients) from Slackware to Ubuntu. What motivated me at the time was ease of maintenance (apt-cron, apt-cacher, etc.) and the availability of nearly every package under the sun.

That system had so many hiccups that I had almost daily phone calls with complaints. This lasted for a few months, until I decided to wipe everything at the following holiday, only to put Slackware back again on every machine. Since I've put up my own package repo, the only maintenance task I have to do is keeping the machines up to date and changing the toner cartridges on the printers. That's what is meant by "stable".

I totally see the limitation of my perspective, I think that in an environment like a corporate server room, or an educational institution a lot of trust has to be placed in the OS distributor. In the case with Ubuntu, they have a pretty large pool of contributors to their repositories. That's a lot of individuals to trust. Stability across that entire pool would be difficult, if even possible to maintain.
Since Slackware has a much smaller pool of "official" repository it would be easier to maintain that stability, and since your Microlinux platform is relatively controllable (I presume), your overall risk for instability as an admin is lower.

With a large enough Hard drive, I wonder how stable Debian would be after some sort of `apt-get install *`?

I think that's the crux of it. Zero defects is an unattainable state of perfect grace, but it can be approached with a combination of testing and testability.

Testability is maximized by reducing the number of packages, reducing the number of dependencies between packages, reducing the rate and degree of package change, and adopting only packages which have already been subjected to a lot of testing.

It's trendy these days to use the term "exponentially" to mean "a lot", but it also has a precise mathematical meaning. I use it in the latter sense when I say that the number of defects in a system increases exponentially with the number of dependencies between components. The other factors I mention have a linear impact.

This means that as the number of packages increases, which increases the number of dependencies between them in an unfortunately complex way, the system's testability degrades very rapidly.

Red Hat and Debian might have more people testing their system than Slackware, but they also have to support a lot more packages, each with their own complex dependencies. I honestly can't see how the greater manpower can make up for the increased complexity.

I ran debian for a year, both Stable and Testing. I found Testing to be more stable than Stable... that was unexpected. This was when Testing was Jessie and Stable was Wheezy. I actually found Testing to be very stable. Of the Linuxes I have used, Slackware and Debian were the two most stable. I run Slackware now because I feel more in control of my system.

Your experience was what it was because of the fact that Testing and Unstable were frozen. Testing and Sid were "frozen" so no new packages were being pushed in from Sid/unstable. This prevented the rapid changes most commonly seen in Testing and development branches. Debian freezes Stable, Testing and Unstable right before a new major release cycle begins. The only updates that occur to any such branch in Debian during these freezes are critical bugs and outstanding security patches/updates. No new software versions or new package additions were being added during this time.

In comparison, try using Debian right after a major release. You will see large instabilities in all branches. In this particular last major release, the largest change that occured was systemd (and all relevant dependencies). So the freeze period was much longer than usual before stable became old stable and testing became stable.

My comments are not meant to bring up systemd. Just clarifying why pzognar's experience was what it was.

pzognar, the debian 'stable' branch means the 'least often changed' branch. stable in this case does not mean 'the most reliable' or even the 'one with the least defects'. It simply means 'least modified'. I haven't read through this thread, but I'm sure it is filled with this same misunderstanding. When I saw the thread title, I immediately thought "define stable".

Stable, in my opinion, means several things:

1. Packages are tested and proven reliable.

2. Packages try to stick to using release only versions of software rather than using git/svn/cvs/mercurial pulls or alpha/beta/rc status software.

3. Packages work as intended, perform well, but do not crash without valid reasons.

To me, that defines stability.

A stable distro will allow upgrades between major releases without pain and major effort.

I do not know if Slackware is the most (stable), but it is probably one of the simplest and also among the least problematic, but nobody is perfect ;)

In my experience debian stable is exceptionally stable, comparable to slackware -current :)