Even though I'm a diehard Mint user nowadays, I agree with this.
I started out with Slackware, and I used it for 8 years before moving on to Ubuntu, and finally Linux Mint Debian Edition.
Slackware, while it has a learning curve, is also (as odd as it may sound), actually quite simple. It does what you tell it to do. No more, no less.
It's rock-solid stable.
It forces you to learn about how Linux works, because you have to tell it what to do and how to do it. It isn't as much work to get running as Gentoo, but it makes you think about things like kernel versioning, what's going on in /etc and where your system logs are, and how to compile applications from source from time to time.
I've taken what I've learned from Slackware and found that it's applicable to every other Linux I've knocked around.
I use Linux Mint more like a "casual desktop user" these days, but if I need something rock solid stable and reliable, I will go back to Slackware, because I trust it. It's not a Cadillac like Mint is, but a stock car that has everything accessible and tweakable, so you can bend it to your will and it'll serve whatever purpose you have in mind for it.
So, to sum up, while it doesn't sound like a newbie distro, I still think Slackware is a great way to cut one's teeth in the Linux world, especially if one is truly setting out to learn Linux, not just using it as a launch platform for a browser and an email client.