I expect the difference you're describing was partly due to moderation (and lack thereof), but also partly due to the barrier to entry imposed by the forum signup process.

Unfortunately, the signup barrier cuts both ways: Despite loving high-quality discussion forums, I seldom bother participating in them these days, mainly because jumping through signup/captcha/email-validation hoops and then having to maintain yet another set of credentials for yet another site, forever, became too much hassle once I had more than a couple dozen. (I have hundreds, so I'm very reluctant to add to the pile.)

OpenID managed to solve a good deal of that hassle, but it's mostly forgotten these days. I think well-moderated federated services have the potential to solve it completely, though. Here's hoping.