Todoist Premium offers you many useful features such as:
- email or SMS reminders,
- adding notes to tasks,
- sending tasks via email,
- calendar export via iCal feed,
- task search
- full completion history with graphic visualization of your productivity.
Feel free to take a look at this video: http://www.youtube.com/watch?feature=player_embedded&v=IWCBVE3IhJ8 to see what Todoist Premium can offer.