When some new regulation or law comes out that requires additional data to be added to the system, with a capture point in an end-user application, and an export point to an external system, and you can implement the change in under an hour, you have designed (at least that part of) the system well, IMO.

Edit:  By "under an hour" I mean it took me more time to open the project than it did to make the change.  I feel productive!