Things, a task management app that's still in development, does a great job letting users know about its unimplemented features. If you try to drag a task from a project into the 'Scheduled' area, for example, you'll get this popup:
Every action I've performed in Things has either worked as I expected or popped up one of these helpful messages. That's awesome! It feels good to know that they're moving in the right direction and makes me more confident in their ability to make a great product.