1. Points based permissions. This would curb malicious activity and cheating the system. At least have something to restrict rampant voting (as I described here)
2. Flagging/reporting system.
3. Closing/deleting questions and users. Closed questions should have a suffix, i.e.: [closed]
4. Time formatting. Allow us to choose what dates look like. I want to use timeago which brings me to..
5. jQuery. Ajax updating, inline comments, posts, etc.. Popup UI dialogs, plugins which make the whole experience much more pleasant and interaction easier.