> I think a part of this is how forum members tend to populate avatar/signature/location/etc and the interface shows it prominently. On reddit you just see a username and occasionally some flair, so you rarely even realize that what you're reading was written by someone you've read from before.

I don't think that's it. I'd probably attribute the anonymity more to size and the deliberate lack of friction for someone to move between subreddits.

To get a sense of community, you need a space where you commonly interact with the same people over and over, not a fire hose of different ones.