My son, Roark, is 6 and loves his Legos. This is not an over-statement or hyperbole. Every single entry on his letter to Santa this year was a Lego set and our most effective 'carrot' for good behavior continues to be access to his Legos. In fact, this picture was taken just this morning on the floor in his room and yes, that is an industrial sized storage tub full of them...
A recent NPR story on Legos gets to the heart of it. Legos help my child express his 'story' - whatever his story happens to be on any given day - pirates, ninjas, cops and robbers, school, etc.
Despite its patents expiring some years ago, Lego continues to dominate in its the market. NPR reports it is 70% with its closest competitor, Mega Blocks, at 30%. Legos has made many smart business decisions -- including exclusive licensing of Star Wars, Toy Story and Harry Potter experiences. These popular branded experiences appeal both to kids and to the adults "helping" them to build the sets.
While these licensing deals are key for differentiation and demand, I would also suggest that Lego's longevity and success also has a lot to do with their commitment to their intrinsic brand promise: that every single Lego piece fits with every other Lego piece. NPR reported that even their manufacturing process is designed with this in mind. They imprint information onto every brick to make it easy for them to identify and correct any 'bad' batches so the consumer only has positive experiences building with Legos.
Because, as any parent of a Lego obsessed child knows, every set that has ever been painstakingly built for hours will sooner or later be dismantled in order to create something new. And that something new is more than likely something from the child's own imagination. And Lego's ability to continue to allow my child to play and create even after his interest in any one set wanes...well, that's something I'll keep paying for!