One thing I really noticed in the migration to journal-based fandom is that people started to think of fic length in terms of how much will fit into one post (especially since on LJ it's roughly 10,000 words, which is a nice neat number).

Huh. At the time I definitely didn't have my finger on the pulse of fandom to be aware of things like that. I can understand being aware of how much will fit into a single post, but did writers really curtail their writing (consciously or unconsciously) based on that word limit? Did you? I was actually thinking of flashfic, challenges like contrelamontre, and the rise of the drabble. I would have said there was definitely more short fic being written, but I just thought there was more fic, period; I didn't think it was necessarily being written in place of longer fic. Personally, I would consider a fic that was 50,000 words to be long no matter where it was posted. Perhaps livejournal made us more aware of things like length and wordcounts when we might not have been before?

I'm in a stage right now where I don't have a primary fandom

That seems to be going around lately. It feels like a lot of fandoms are winding down, but nothing's really winding up.

I also have a WIP tag that I check whenever I remember to, which can lead to moments of "OMG YAY THERE ARE FIVE NEW CHAPTERS."

Haha, yes! It took me a while to figure it out, but the secret to happy WIP-following is to FORGET ABOUT THE WIP until it's updated again. \o/