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For the record I love slackware and still have it on my work desktop and other assorted servers at work. It just works. That said, I also moved my home desktop to gentoo over the weekend.
Slackware's documentation is appauling to say the least. Documentation should exist for nearly every potential user function. Take a look at Gentoo's documentation page, and you'll be impressed.
I have to agree there. Gentoo's documentation is great. Probably part of the reason of Slackware's lack of docs is that slack has been around for so long that the old-schoolers just already "know" how to mess around with /etc to configure stuff. I like the env-update feature of gentoo. When you talk about gentoo documentation don't forget the 800+ people that are on #gentoo at irc.freenode.net at any one time and also the incredible forums. For a fairly recent distro, gentoo seems to be gathering a lot of followers and your questions will be answered promptly on the forums.
It's either non-existent in slackware, or the available ones hold no water. I tried installing GNOME on slackware and it only brought me close to tears. It was diabolically frustrating for no sound reason. Why should I bother haunting dependencies down all over the internet, when an intelligent package manage will not only do that for me, but will install the package in a consistent manner over my file system.
That's why you use swaret to handle your dependencies and better yet use dropline to install gnome. I hear there is a slapt-get, but I haven't used it.