I think the ability to delegate tasks to Non-Doit.it contacts would be extremely useful.

I imagine the feature would utilise email to send tasks to non-doit users and provide mail links in the message. One for accepting the task. One for status updates and one to mark the task complete. You could look at how GitHub enables users to respond to posts via email for existing similar functions.

The doit.im user would be able to import contacts from major email providers or csv.