The Slackware documentation has a summary on what makes it stand out:
In other words, it really doesn't have a lot of inconveniences after all. I think the biggest reason I moved to Slackware in the first place was the glut of dependencies that were installed whenever I installed a package in Ubuntu. With Slackware, you start out with a good portion of the packages you need, and manage the rest when you do third party installs. And while that may seem challenging, it ends up being fairly easy, since once you have your install set up and customized the way you like it, you can run it for years without having to make any drastic changes.
Also, the packages are all plain vanilla software, with very few distro-specific patches. While this tends to make the distribution seem less "uniform" out of the box, you also end up with more stability.
Full version upgrades also tend to be easier and more stable overall. Granted there's more work done under the hood, and there's always a chance you can mess up, but I've found that every time I've made a mistake, I've been able to rectify it using some simple method.
And that brings about the most important aspect of Slackware. It's the distro that puts you the closest to working with Linux, without having to delve through layers of "convenience" UI. It may seem harder at first, but after a bit of learning, you'll know Linux better than just about any other distro. (Excluding Linux from Scratch.)
That said, Slackware isn't for everyone. If you just want a distribution that takes the minimum effort to get going, you're probably better off with some of the other big names. But if you have the time and a bit of spare hard drive space, I recommend giving it a try nonetheless. Just be patient.