Usually, people would complaints when something goes wrong or didn't work. But it may not be the condition when you talked about Slackware users (or some people would say Slackers). Some of them complaints when everything has worked out of the box, where they expected that they should have "hacked" their system to get it working
Check this thread at LinuxQuestions: Complaints about Slackware 12.2. With new kernel and better device management system, most Linux distribution now works out of the box and can detect most devices (except for special devices or newer devices which has been released after the kernel has been released) automatically. But note that not everybody likes this, because there's no adventure of hacking their own machine to get the system detected the devices. That's the beauty of working on Linux
Let's say you use Ubuntu because it is usable out of the box, meaning that you don't need to set up anything for general use. You *may* not like Slackware because you *may* have to set up things by yourself to get the preferences you prefered. It's just an example, and should not be taken as general rules, as there are people who loves both distribution.