Like some other reviewers here, I´ve seen my share of big cathedrals, and sometimes they seem to blur in memory. What´s unique about the one in Amiens ? Not that much, although it is very big and definitely beautiful. I just can´t say why it would merit WH status, since there must be at least twenty very similar cathedrals in France alone.
Having said that, when you are anywhere in the area, you should go to Amiens, since the town is very nice and pleasant and it gives you the benefit of seeing three WH sites for the price of one, so to speak: besides the cathedral, the belfry, which is just a few minutes walk from the cathedral, is part of the "Belfries of Belgium and France", and - something I only noticed on a plaque inside the church - the cathedral is also part of the "Routes of Santiago de Compostela in France"!