Three design principles from Lego CEO Jorgen Vig Knudstorp:
When it’s advertised does it make a child say: ‘I want this!’?
Once he opens the box, does it make him go: ‘I want more of this’?
One month later, does he come back to the toy and still play with it? Or does he put it on the shelf and forget about it?
I believe the last point is the most important. Although it should say six months, not one.
(From Brickipedia: the story of Lego, Saga Magazine).