[AgentME]

Reddit and HN don't bump threads on activity. HN actually penalizes threads with too much activity. The anonymity is a big factor too; I think it's the combination that helps make things bad. It could also has to do with the way replies are shown: Reddit and HN's branching style causes discussions to fork off in a hundred different directions and focus on different details. Classic bulletin board forums make it difficult to really follow a thread as it gets too busy as you have to click and wait to load a new page for every 10 or so posts. Imageboards often show replies in a single quickly-scrollable auto-updating page in a very compact manner. This might make bandwagon effects much easier.

Maybe I'm wrong about how the specific details play into it exactly, but I think the differences between site structures is not considered nearly enough when trying to understand the differences between site cultures. I hope it's apparent to most that Reddit+HN, classic forums, Twitter, and imageboards each strongly influence discussions to work in different ways, and I don't think it's just because of their different communities. I think if you swap out the people or make multiple sites with the same structure, you see that each structure reinforces its own set of behaviors.

>perhaps anonymity has the opposite effect, allowing people to be more willing to have thoughtful discussions and change their minds, instead of having to stick to their guns for fear of losing face.

I can see the logic of that, but the "thoughtful" part has rarely been my experience on any anonymous places. I think people are more willing to change their minds, but in the direction of being more willing to change their mind to follow the "hivemind"/community or change their mind in a way that's more able to provoke others.