yeah the new dawn of the dead is probably my favorite, but I also really love Night of the living dead.
Thats easy decomp, pure and simple the more decayed the re-dead/zombie the more limited it's mobility, for example take romero's films(I'm using his works as a example since he invented the modern zombie film) in night we had two types of zombies the reanimated long dead corpses and then you had the recently dead, a zombie who was reanimated after being dead for some time(these would of been the first ones) there bodies would be stiff from rigor and the muscles would be weakend so they are less mobile while the more recent dead(those who died after the incedent began since in Romero's films all able corpses reanimate) unless they suffred from pre-death condition such as a torn leg.