I couldn't agree more with trenchantly and philrob, and I think Omni Clint misunderstood Lizard.
"Defer" means to put something off that was already scheduled. It does not mean "start" (at least in my experience.) If I was in a meeting, and we discussed a new project, and someone said "When shall we defer it" or "what defer date shall we set?" I would have no idea what they meant or why they were using that word. This is how it seems to me (if the functionality of Defer is the same as the old Start field—if wrong, please set me straight.)
If this is correct, then I would agree that a "postpone" or "defer" feature would be great. This would easily allow a user to postpone or defer the due date (I know, not very GTD, it is not unusual for me or my clients to postpone a due date of certain tasks).
Along these lines, it would be nice, if, in Forecast, projects/tasks that have passed their start date, but not yet reached the due date, would show up. That way, a quick glance at any day in forecast would show tasks that are DUE, and tasks that have been started.