What do you want to see in a social task management app/service?

I'm working on a social task management system, and I'm wondering what you guys would want to see in such a system.

Basically, it would let you create tasks and share/assign them to people (but it goes much farther than that).

Do you see yourself using it? As an advertiser, would you pay to make your advertisements into actionable tasks that people could add to their to-do list (or share with friends)? Would you assign tasks to your friends using it instead of asking them in person/by phone/by email?

I'm trying to identify as many use case as possible, so don't hesitate to share them here.