I was sitting and thinking today, "Why is it so difficult to find the reply I sent to that person?"
Most e-mail clients track who you have replied to and, almost always, that message is sitting in the "Sent Items" folder or something similar. GMail includes all replies to a message in the view but I don't really like that. If an e-mail client can track what e-mails have been replied to...how much harder is it to also store a pointer at the message that was sent as the reply? Then you could right-click in the e-mail client, click a "View reply" menu item, and immediately jump to the reply message. If you replied more than once, a dialog box would pop up asking which reply to jump to.
This feature could be implemented in probably 30 minutes. Yet no one has bothered to do so.