There isnt as much difference as you'd think. Does Apple extra engineer some of the acessability features such as the side-loading opening they had on the G3-G4 towers? Sure they do. But other than that its all components. They're main design win is the case and I have a feeling most PC case manufacturers are just afraid to overengineer in convenience like those since they have no control over how high or side the CPU cooler will be that the user gets. Or exactly where the RAM will be and how high a clearance they need for it. Apple has extra comfort of knowing all these things going in since they have one motherboard going into it. Then if they want to reuse it for another machine they design the motherboard around the limitations of the existing case.
That said, you generally always saw no personality in PC motherboards. A bit of personality in Mac, and tons of personality in the real old school machines like Amiga, Commodore, etc. Names silk screened onto them, etc. Last MB I saw with some style was a G3 from Apple with an Apploe silkscreened onto the MB.
