Forums and then mailing lists were my first online fandom; forums but more especially mailing lists fostered inward-looking communities that developed common cultures, and long discussion threads; they were also a safe place to post fic and receive comments, often posting in parts, often as works in progress. There would also be group writing events similar to today's Challenges, and the occasional round robin. I was on two lists for the same fandom, one of them general and open to all, the other more moderated and limited in topic. The smaller one felt much homier and was where I spent more time. The more private the list, the more free the discussion, in general.
Forums and mailing lists were a vast improvement over print newsletters and Letter of Comment columns in zines - instead of replies to discussions taking months or years (or never happening), discussions could happen almost in real time!
The migration to LJ (and other journal/blog platforms) was a huge dispersal - you might or might not have been able to find people you knew before due to changing handles, not knowing they had a journal, etc.. If you didn't know them personally, and they had a private journal, you would no longer be able to access their input. Friends lists and communities helped combat the dispersal, but the discussions were still no longer centralized in the same way; you might have to travel through a number of journals to follow a single discussion, and you might not know where all the pieces of the discussion were. This is even more complex because some fans went to other, smaller platforms, making them hard to track down or find. I think journal fandom requires a lot more curation on the part of the individual fan.
The move to LJ did make things much more open! And you could find new people with whom to interact! Which is awesome!
Tumblr, I think, is an extension of the same issue, only now it's even harder to have long-running discussions; it's more visual, and more "here's an idea." Which is still fandom, and still fun, just different.
So far as today's archives go, I love AO3 - I feel the centralized nature is extremely beneficial to fostering community.