Since I discovered Slackware I will never go back to Ubuntu or Debian.
What are some of your main reasons?
The two last reasons are so much easier and faster to do by hand and it was such a breeze to do in Slackware compared to when I ran into it in Ubuntu.
The second thing that I like a lot more with Slackware is the config style; to go into a file and to edit it directly instead of doing it GUI-style. I guess you could configure it in the files directly in Ubuntu too, but Slackware is basically built around editing these files for configuration and so I've find that they're a lot easier to edit; great documentation and easy to find.
Ubunut is a great way to easily get a working linux up and running without much configuration needed, but if you have the time and energy to sit down and tinker with it a bit like I had then Slackware is pretty easy and straightforward too.
Lastly I think that if you run Ubuntu, you will learn exactly that: Ubuntu. But if you run Slackware you will learn more linux with only a few Slackware specific parts like the startup scripts and the package tools - I can't honestly think of anything else which is unique to Slackware (the startup scripts are actually BSD style but meh). Which is a good thing if you want to learn linux like me.