constraints and affordances is at the heart of the “flat design” vs. “skeumorphism” debate. the benefit of skeumorphic interfaces is that replicating the look of real world objects like buttons, provides a natural way to communicate interactions. where skeumorphism went wrong is communicating false affordances: a detail in the ios6 calendar app hinting that pages could be torn out- when no interaction supported it.
flat design throws the baby out with the bathwater. we still need real buttons.
5. Habits and Spatial Memory
this is mostly about not arbitrarily moving around.buttons in an interface. people are creatures of habit, and if you fundamentally change the method of performing a task for no good reason, it’s not a “UI revamp” it’s pointlessly frustrating your existing users.
for spatial memory, millions of years of evolution have left us with mental machinery for remembering exactly *where* something is physically. you can take advantage of this in UI with persistence of space.