A todo list is one function of email. It's also an information sharing and socialization service. The right solution will contain specialized functions for these various uses. Note that a single email can fall into multiple categories. "Prep for 12/3 meeting" might contain information which should be added to a searchable library for future reference and create a to-do entry to ensure that the documents are read before the meeting.

The bigger challenge is that e-mail as a to-do list differs from a calendar in that the time and place of the to-do's is a bit more nebulous. For example, if you send someone an email with a link to your page and request feedback, you have a certain but inexact timeframe in mind. The secret to making this work is providing a heuristic ordering based on the sender's desires and the recipients priorities. The former can likely be specified with a few request classes or duration options. The latter would likely have to be learned from usage history, possibly with a VIP list used as a starting point.