It would be cool if you could tag users in a post. Much like how the "reply" option works, but you could do it for people who haven't commented on the thread yet.

For example, I'd love it if someone could just type "@Adira" in their comments, and I'd get a regular Reply Notification e-mail about it. Sometimes people ask questions and you know of someone else who has experience and you want to call their attention to the thread - being able to "tag" them would be a good way to do this.