Fitt's Law - size and position!
Hick's Law - 7 plus/minus 2!
Ok, so I’m not saying that we need to know the maths, but the underlying principles are important! Fitts’ Law tells us that the time to acquire a target is a function of the distance to and size of the target, so the size of a link area and it’s position on the page (and relative to other elements) is important. Hick’s Law says that the time it takes for a person to make a decision is a result of the possible choices he or she has. More choices mean a longer decision process – which gives us the 7 plus/minus 2 rule.
These are two of the fundamental rules that I use in user experience architecture. I’d recommend Tog’s (Bruce Tognazzini’s) First Principles of Interaction Design for some more.
Regarding Hick’s Law -> Barry Schwartz: The paradox of choice http://www.youtube.com/watch?v=VO6XEQIsCoM
Fitt’s law is essential knowledge for any gui designer.
One of the 3d modelling packages (I can’t remember which one it is though) shows a great example of this: when you right click you get a star shaped context menu so that all menu options are equal distance away and hence all equally quick to get to.