You can now Tag people in comments! Just use @username and they'll get notified.
I only got to spend about 30 minutes making the feature so far so it only works for comments but I wanted to share it so you can start using it! I'm planning on adding it to post descriptions as well and then upgrading the system to be smarter and better as time goes on if it gets used often.
Just say @username in a comment and replace "username" with whoever you want to mention!