A lot of the time when I log on my small private server I notice that something has changed and I want to know who did it and why. Also, I might build something that needs to be finished and I don't want anyone touching it until I'm done. There should be a sort of journal, not a craftable item or anything in the game, just a button in the pause menu, that lets you mark down notes. Other people could come in and view the notes, but they don't have to if they don't want to. It would be public to read, but you wouldn't be able to edit other people's notes. You could also keep all (or at least a lot) of the notes on record so you could scroll back through them. It's not very complicated, and would be pretty useful for communicating with the rest of your team if you all aren't on at the same time.