Moreover, most MUAs can be set to ignore the X-Priority, X-MSMail-Priority, etc. headers altogether. Everyone's email looks the same to me, so a few times a year I get to take an angry call from someone asking why I haven't replied to their urgent priority message yet. Sorry, but "Subject: data export for October" isn't as urgent as you think it is, and the guy who sent "Subject: Utility work at Chicago datacenter tomorrow" with a normal priority header is actually going to get attention first. Outlook's return receipt feature is another barrel of fun, my client is set not to honor them, which occasionally sends people into a frenzy.
Speaking of things that have been done well for decades, the @mentions thing doesn't sound like any great shakes either. "Making it easy to identify emails that need your attention, as well as flag actions for others" sounds a lot like CC'ing people to me.