I think the comment-reply (ping) mechanism is way too limited. You can only reply to one person in a comment. And you can't use an @ whatsoever in questions and answers. It just doesn't make sense to me.
Sometimes, when responding to a chain of comments, multiple commenters might be making the same point or similar points, and your answer to them might be the same. Therefore, it would be better to be able to comment-reply to all of the users in one comment, instead of posting multiple comments or only responding to one of them.
On a question asked before about this, user Manishearth answered that one can just add a comment under with the other user's name. Referring more than one user in comments Not only is this less practical, but a few problems can arise.
Firstly, what if there is a lot of people you want to ping? If the comment chain gets too long everyone is moved to chat, which might not work too well in every situation.
Secondly, in a case of a sea of comments, your extra referral might get lost in all the comments, leaving the user tagged oblivious as to what comment they were actually tagged to.
In questions and answers, there are many times were pinging someone would be quite beneficial. Just look at this question, where notifying user Manishearth of his mention would've been great, as I'd be able to get his opinion, and he would be aware if I was using his name in vain (or just misrepresenting his opinions or answers).
If your question includes or links to a post of another user, then it would be great to be able to ping them. Or if the question is about something a user said, like this question: What constitutes as etymology within English?
And in questions, there are many times you find yourself quoting or pointing to something another user has already said. What if we could instead of just give credit to them, also let them know we referenced something they said first.
If there is a reason why this wouldn't work, or would be bad, please enlighten me.