I would like to suggest a new idea which i have partly implemented in my website as well.
Since I'm a wordpress user one main feature that i feel as lacking is the mail list with tracking.
My idea is that, once we post a article in our blog, we should be able to send that to the viewers or subcribers who subscribed to that category in email. Also Once the user opens the email, we should get an notification regarding that. It means we should be able to view how many users read our email. By this tracking system we can be able to manage the mail list effectively. I have tried the tracking system as a standalone web based application and i have suceeded on that.
I'll really love to see this feature in wordpress, since i like to use wordpress for all kinds of needs in a website.
Please visit my blog for more clear details about the mail tracking system.