the recently remodelled staircases in the Illum store in Copenhagen
Well, actually, quite difficult.
A staircase is not just a major feature in any building but it can also be a particularly difficult part of the design to modify if other parts of the scheme are changed as the plan develops. It becomes a difficult game of consequences - change one part and another no longer works.
It might sound like stating the obvious but a staircase really does have to function well. A doorway can be slightly too narrow or a window sill too high and people grumble. If a staircase is too steep or too dark or the steps are irregular or too small then it is difficult to use easily and it might even be dangerous.
A design for a staircase normally has to start with the dimensions for the height and depth of a step - the tread and the riser - fixed by the average foot size and the average stride pattern so a tread of at least 300mm and a step up or riser of between 150 and 200mm. These can vary slightly from one staircase to another but not by much and they have to be consistent and ideally consistent through the full height of that staircase. Just watch how many people stumble at the top and bottom of an escalator if it has stopped moving so after a number of abnormally high steps you get into a rhythm for the stride and then hit two or three very shallow steps at the end. It is interesting that even though people clearly understand the escalator has stopped many still stumble.