No, there are means to handle that as well. For instance, I could just invoke their username and let the software handle the linking and notification. Perhaps i

No, there are means to handle that as well. For instance, I could just invoke their username and let the software handle the linking and notification. Perhaps it needs a trigger like @ or not to separate regular conversation from usernames, but all the primitives are there. Facebook has some pretty decent tools in this area, so does Disqus. I'd also mention that there's a difference between using a special character as a trigger and building a UI around it. Its the latter that I'd argue Twitter needs to deal with as the current presentation is overly complex and not nearly relevant enough for the vast majority of users.