What would be awesome and truly useful is real constraints, like in CAD software. I should be able to express every width, height, x, and y value as a variable, or as a function of other variables. So I call the device height "A", the width "B", and express the rest of the UI in terms of those variables. Then at runtime "A" and "B" are defined, and the rest of the layout magically does its thing.

What Apple calls "constraint-based layout" is cool, but does not follow the classical definition of what a constraint-based layout actually is. They don't use variables anywhere in the IB UI. You can only define proportions in code which is incredibly clunky.