This may just be a massive change the site simply isn't equipped for, but I figured I'd throw it out there as a suggestion anyway - some group function like LJ communities/Facebook groups would be great!
It is nice to be able to post to a specific audience. It would enable social interaction and conversation without private messaging. Rather than having to scroll back through global tags, people could search posts within a community. A lot of gripes folks have with "discoverability" and tag syncing could be alleviated a bit I think. (People who feel annoyed/conflicted/whatever about tagging #photo #photos #photography #digital photo #digital photography #picture #landscape could post their photos in the Photography group instead.)
I often see people on Tumblr for example despair about posts "breaking containment" - they mean it metaphorically because "containment" is purely imaginary there, and it's easy for posts intended for a specific group or subculture to explode out of their intended context and turn into fights and harassment. This happens on Twitter a lot too. (For example: someone with a pet spider has no way of posting pics of their spider exclusively to a Yay Pet Spiders group, and instead simply has to hope that their spider pics don't spread outside of their social circle into the broader public where hundreds of people will comment saying things like "i want to kill your horrible ugly pet.")
Communities/groups can be a great way to organize more general static posts like tutorials, link roundups, informational posts, etc and keep them from getting washed away with new activity, which happens in tags - if there was a "cohost newbies" group where someone could pin one "welcome to cohost!" post and others could share their welcome-to-cohost tips, that would be helpful, I think!