I agree with most of what you said, except that the real-time aspect is not real-time enough. If email were actually real-time, then it could completely replace instant messaging as well. Right now it's lacking.
I think gmail's "conversations" are really the first try at what I'm talking about, a new front end on email, but I don't think they work well enough personally. And they haven't really expanded well on the concept recently.
forgive any appearance of snark; i just wanted to be concise, and i don't think that "the ideal marriage between email and IM" is something that people were - or are - desperately crying out to have.
1) Busy, complex interface.
2) Sluggishness.
I find Convore to be a much better way to approach the same problem of how to enhance email.
It doesn't allow interactivity (which is what you're probably thinking of) but it does provide real-time content.