I was just browsing through this paper on "Generating summary keywords for emails using topics" and was fascinated by its usefulness, especially because it "selects words that describe each message in the context of existing topics rather than simply selecting keywords based on a single message in isolation."
Imagine if we had a smart email client which will automatically show the summary of the email in a few words rather than simply junk/not junk classification. That would help us a lot to triage our email.
On another tangent, how about an email client that sorts your inbox by importance and not by time? Importance can be automatically determined by how often you reply to the sender, what topics you reply fastest to, whether the sender is from the same company, etc.
We have websites like Amazon and Yahoo! that automatically customize their websites based on our usage patterns, why can't email clients do the same?
Email clients have been around for so long, can they become smarter than a sorted grid with folders?
P.S. While email is the big fish, there is a lot that can be done for personal information management.