Deleting abandoned accounts, and what constitutes account abandonment. I get very strong violent reactions to the mere suggestion of someone deleting an abandoned account because they want the motherfucking username, without the permission of the owner. The absolute only way I could see doing this is making the archive available to the authenticated previous owner, and even then, I'm reluctant. Very reluctant.
Crossposting is necessarily a bad thing. (LJ isn't built to handle it, but if LJ were rebuilt from the ground up, it might not be a bad thing.)
Merging two journals would be REALLY REALLY AWESOME.
There's a really strong feeling that a lot of stuff about people should be public, even though a lot of people want a lot of them private, like the profile, and icons. Why is there this strong feeling that it should always be public? What does it matter to someone who is on the same journaling service?
Oh, and community memberships being public.
For a service that's got whole bags of very nicely granular privacy, there's a very strong user-drive for some of the vital nuts and bolts to be public.
Adult content flagging that doesn't mention "adult content", to avoid tripping stuff.
It doesn't matter if they're not going to interact. It will matter if they do interact, and here is how. (IP address-based blocking. Doesn't work for viewing, but I don't care if they use it for commenting in a personal journal.)