I browse these forums (OT, P&N, L&R, now Discussion Club) as well as Reddit and 4chan. For me, each of them has its own utility and provides a different experience. I tend to envision them in different ways:
AT forums: usually reasonable people (not so much in P&N), clever, funny, probably a bit older and wiser demographic than Reddit and 4chan. I picture most ATers professionals of some sort, often business types, but sometimes blue collar folks too.
Reddit: about the same age demographic as 4chan (which is like 12 to 25 or 30), but instead of being disgusting and pointless, Redditors are usually just boring and arrogant. I picture them as the slightly-above-average kids you knew in high school. They weren't really hot shit, but they wanted to be hot shit. They think they're way funnier than they are, but they often post interesting and funny things. It's not incredible content, but usually worth a lurk.
4chan: disgusting and pointless is probably a fair assessment. Just a cess pool, but sometimes in that cess pool there's brilliance and human connection you can't quite get elsewhere. They were the weird kids in high school, the ones nobody wanted to hang around because you just didn't feel comfortable around them. They have some shining moments and a lot of potential, but when they're all together, it's just a shit fest and that's all it will ever be.
I guess I'm kind of weird, but I like to lurk in all of the places. I post the most by far in AT (which is saying something, given my posts per day is very low). They give me different windows into humanity. If I wanted to feel like part of a community and post frequently, I'd definitely choose AT or a particularly good sub-Reddit. I suppose some of the 4chan boards could be reasonable too, but I haven't seen it much so far.