How much have you used {Free,Open,Net}BSD? I find the experience to be completely different from that of using any of the Linux distros I've tried, and I'm wondering what you find similar about them...
Slakware historically has been closest to a BSD feel of the Linux distros
Maybe the init system in the 90's was similar when Slack was compared to contemporary Linux distros. There is very little modern resemblance. I wish this perception would go away. Spend a day with both and I'd put money on an honest person never making this claim again.
Late in reply - I switched off from Slack to FreeBSD around 2006. Perhaps things have changed, but Slackware generally followed the BSD conventions of what goes where as opposed to say RedHat.