The rich media thing: it's useful. A minimal markup (such as markdown) would likely be sufficient. The ability to be able to emphasize and strongly emphasize text, create bullets and numbered lists, indicate (multiple levels) of quotes, hyperlinks. There's not much that Markdown itself doesn't provide.
A small amount of chrome for the masses would be nice. For the most part my feeling is that existing HTML markup goes too far, generally in the direction of unusability for far too many websites.
Another feature which is quite useful is indicating followups and actions. Here I find Reddit has one of the best models around, though the UI/UX lightweight nature of G+'s Notifications widget is also nice (the absolute uselessness of most of the actual notifications, and the inability to filter / set precedence / classify types of notifications is a major failure of the site). G+ also suffers from one of the largest DOMs I've seen in any major website -- I've backed off using it pretty much at all, and find my browser memory footprint is vastly more bearable.
I remember forté agent and thunderbird doing it about ten years ago, and I recall being annoyed that markdown differed :)
Formalizing a basic level such as markdown would be a big win.
Also most (American, anyway...) clients couldn't support such large messages, since the choice of speed for most of us was 16.6, 28.8 and 56k modems at the time.