I would say that perhaps, at least from what I've seen, it's often not really a discussion at all, but more like a question answering forum. I suppose what I like about HN/reddit style forums more, is that there is a "post", with a clear topic or article, and then people actually have a full, long discussion about it. Sometimes of varying quality, but it's at least full of threads where people react, people react to those reactions, etc. Whereas what I've seen on IRC is mostly just "how do I do x", and then some random short-lived arguments about how to do X better, or why are you doing X, etc. It doesn't seem to be a discussion but more like a problem solving brainstorm with a bunch of people who aren't stakeholders in whatever the original poster is talking about.
So, I'm not sure, but maybe the way the discussion in a link aggregator is specifically oriented around a "post" and clear topic is what makes it more engaging for me, rather than just a nebulous promise that "this channel is to discuss X.." followed by people just sitting there with not much to say about X.
But, maybe my take is completely wrong, and that's why I asked the question, maybe I've missed the very interesting and dynamic back-and-forth discussions and funny moments that attract and keep people engaged in a chat room. Maybe it's just a matter of being there long enough.
