Very good idea... I have a few suggestions...
But to get started we have set this up on our website (www.iasdsydney.org) but doesn't send email reminders...
1. Description Field (to add more information as email subject)
2. Category Field (To organize under what type of category.
3. Tags Field (Easier to find Tasks)
4. WordPress Users Integration (So we could choose from a list.
5. Page View to Share with friend the Task (Calendar View), Per Categories, Per User, Filtering with Tags.
6. Other Apps' Integration (Maybe an Events App Integration - All in one Events, as tasks of an event)