I have participated in fandom in several contexts: mailing lists, Usenet, discussion boards, and LiveJournal. It's not news that the first three are generally topic-based while LJ is person-based. On LJ, you create your own community out of who you read and with whom you engage.
LJ is personal. Again, not news. Your journal is your space, to do with what you will. Your fanfic, your art, your work news, your pictures of your cat, your comments on politics and TV shows. It's about you, like my journal is about me.
LJ privileges the original post above the replies. In every design I've seen, the post occupies the main portion of the screen, right at the top, and the replies are below it in their own section. They are, simply by their location on the screen, less important.
In contrast, mailing lists, newsgroups, and discussion boards (for the most part) treat the original post visually the same as the follow-ups, putting it first but not styling it significantly differently.
I think this affects discussion. The privilege of the LJ post, along with the fact that this is a specific person's personal space, means that criticism of the subject matter is more likely to be interpreted as criticism of the journal owner, even if that interpretation is made subconsciously. If you do not agree with the poster, on LJ there is a greater feeling that you should just not read their journal. Again, this might only be a subtle pressure, not explicitly expressed.
You are in another person's space, not a shared space. So, like when you are in someone else's home, there are things you feel constrained not to do or say.
Not to mention, because of the privilege of the LJ publishing model, your readership will be made up mostly of people who agree with you. You may be more likely to find flamers than people who disagree with you in a non-hostile way.
Communities do provide a place for finding discussion with non-like-minded people, but the privilege of the post still remains.
LJ is public. The nature of a web journal -- a publishing model rather than a discussion model -- makes it feel more public to us, even if it is filtered and locked. Here's an example of what I mean:
Suppose I went to visit kestrelsan and while I was there, she only ever made me oatmeal for breakfast, even though she knows I hate oatmeal. (Of course this is hypothetical; probably she'd make me get my own breakfast. *g*) After I got home, I really wanted to complain about it to ten of my closest friends, but not let kestrelsan know I was doing it. I could:
* send private email to everyone in the group, either individually or all at one go
* send email to our private and exclusive mailing list
* post to LJ and filter it to only those ten people
In any of those cases, someone could easily tell kestrelsan what I had said about her and her oatmeal. (And there could be technical glitches or operator errors as well.) But which instance feels more public to you, on a gut level? To me, it's LiveJournal, because it's published on the web. It feels more visible and permanent, even though it's no more so than the other two. (A web-based forum or discussion board is also public in this way, even if it is restricted to certain members.)
I'm not sure how this impression of public-ness affects what we post under lock and filter, but I think it affects how we feel when some friends-locked unpleasantness is made known. The offense feels greater because it was in a public space, even though the actual public nature of the information was the same as though it had been emailed.
On LJ, we build our own community by reading journals and interacting with them by means of commenting. I'm sure most of us have journals on our lists that we consistently read but don't comment in. It's interesting for the reader, but it doesn't build community between the poster and the reader. (I frequently admonish myself to be a more frequent commenter, but haven't had much success.)
We post, by and large, hoping to receive responses, those little strokes that let us know that people care about what we say (or, more likely, care that we are still alive). We may also be hoping that those replies contain advice or praise or smart discussion on the topic at hand, but at root, we just want to know that someone is reading. And by replying to those comments, we build relationships and community with our readers, just as we build relationships and community by commenting on the entries of others.
The larger the responsive readership of a particular journal is, the less the journal owner needs to go afield to find community. This is something I just figured out recently. If there are a lot of people responding to what you post, a large part of your social needs from LJ can be met right there, without you having to go out and comment in the journals of others.
Further, you may not have time or social energy to do more than keep up with the responses in your journal. So I think that in a way, a large friend-of list, if it is a responsive one, can be isolating.
And one more thing: We want our friends-list to post interesting things about fandom; we want to post pictures of our cats. Okay, that's a sweeping generalization, but I think it has some truth at the core.
And you, what do you think?