Drag and Drop Capability - If not on-line, then in the app, the ability to move a task from one heading to another (say in the Context View) and have it take on the attribute of the new heading. For example, drag a task from the Home context to the Work context and have the task change from Home to Work.
Weekly View - To see seven columns, one for each day of the week, and be able to drag tasks from one day to the next (not unlike Outlook 2007's weekly task view).
The toughest part of the sidebar is the ability to filter things. If I use Today, I can't sort all the tasks meaningfully. The only thing that is usable it viewing what's starred. For the most part it works okay, except for the sorting of folders. It makes no sense.