I have seen questions I could answer which have laid unanswered for days.

When all the world's experts hang out here, who should answer questions on other forums? ;)

Seriously though, I think it's the critical mass effect. Only when something has reached a certain size and quality, it will attract new users and hold existing members. PM has (luckily) somehow managed to get beyond that point — which isn't all that easy to achieve.

I for one strongly disliked web forums (in general, as opposed to mailing lists) when I first came here. But despite this, I'm still here, so there must be something to it...  Hopefully, the critical mass won't get too physical and blow up one day, or become a black hole :)