I think I agree with you that experiences on various online fandom media vary depending on where exactly you're hanging out (which LJ comms, which forum etc) and what you're doing there (discussion, fanfic etc).

And yet, I've managed to get into more involved discussions on journaling sites than I can ever find going on in forums. Maybe I'm going to the wrong forums? I think the main thing is that on LJ things tend to blossom into much larger topics (for good or ill). I might make a post about Lyra, see, and say something about gender, and then someone gloms onto that and is talking about gender politics in HDM, and then someone gloms onto that and is talking about gender politics in general media, etc. On a forum, people totally go OT all the time, but it tends to be more narrow in my experience.

This is a very interesting point, since I have seen this exact discussion develop along very similar lines! I do think forums have the potential to encourage long, drawn out discussions (although they do tend to focus narrowly on the texts being discussed, rather than branching out into other texts), but it depends on the forum and the people who use it.