When someone is mentioned using @username syntax, there are already some cool things happening, like showing user-vcard on hover and sending that user an info.

It would be even more easy to use, if it would propose matching usernames after the first ... say: 3 letters of a name have been typed; just like the Assigned to or Projects fields do.

Sure, for nicks like Judge there isn't much value in this; but if a user is named thisisthelongestnickicanthinkof, this comes in very handy.