I've been a member on IMDB since the end of '90s, and I've seen high quality discussions and low quality discussions on IMDB. I think it goes too far to say that either is dominant. I can say though that controversial / politics related subjects likely create low quality discussion whereas clever movie plots generate high quality discussions. At least, that's my perception/experience of it. We both got our data point of experience, and that's it. We need some kind of data / analysis to get to a more coherent, unbiased viewpoint.

There's a difference between requiring an account to lurk and requiring one to post messages

I’ll bet that this is primarily not because of registration but primarily because of topic constraints- you can only discuss movies.

And I’m guessing there was some problem, which is why they shut the forums down.