Coming from a UNIX background I love Slackware for the way it tries to stick to the standard UNIX way of doing things.
For me that makes it feel more like home and it is easier to configure right. When the budgets did not leave any room for HP, IBM or Sun systems, there were always the possibility of a stock server (and in some cases, any old PC) and a Slackware install disc.
The slow release cycle and well tested patches, where everything has to be rock solid takes some of the fear of upgrading and patching out of my working day. These days, slapt-get and a few well crafted scripts will see your system running with very little work.
Slackware has a little higher learning threshold than the newer distros but the lessons you learn are very valuable for any *nix system, and it is fun.
I *love* it.
 - for wildly differing values of "standard".
 - In much of the way S&M can be said to be an expression of tenderness. YMMV
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