I think it's the software itself - not the discussion about it, the software. Like someone said, it's "sticky." I find myself checking back here anytime I've got five minutes to kill. Maybe I have an alert! Maybe someone liked my post! And if there are no alerts, I figure I had better get cracking and post some more because - no posting, no alerts. It's an extremely effective positive-reinforcement system.
I've been on vB since somewhere around 2006 and I think I have maybe 300 posts there, and I loved that software and the vB board vibe, right up until the regime change - so it's not just that I love the software, or just that I enjoy talking about software. And I'm not even using the software yet, so none of these posts are of a "how the hell do I do this?" nature.