Better make-up budget. Thats it. As for the chage in ethos from slimy villains to proud honor-obsessed warriors, Gene Rodenberry was retconning quite a bit at the time.
The definitive explanation for the smooth/bumpy Klingons is given in the Enterprise episodes "Affliction" and "Divergence"
Are these the two Klingon Augment episodes ?
I enjoyed both of them, but couldnt relate those statuesque young villains with the long haired psycho honour obsessives in DS9 eg.