Something i used all the time as a moderator in a different forum was counter for new posts (existing or new threads) when in a parent category and viewing the sub category list.
Without this count feature, I have to visit EVERY sub community to see if there are any new posts. Some sub communities don't get traffic that often, and although i could subscribe to these (via email) then i get a lot of noise. I don't want to read every email and then re-read every thread.
In the section that has "pick your community" and the columns for Discussions, Docs, Blog Posts, there could be an additional column for "new" or "unread" -