I'm going to start a running list of The Walking Dead questions for amusement purposes.
Seems to me that if you take Romero's "Night" and remake it into an unending nightmare that has transformed the whole world, you'd probably want to think these things through before you create a comic book or TV series around the event.
1) What fuels a zombie?
Do they NEED to eat, or is it an instinctual holdover from their prior life? Assuming that the Law of Conservation of Energy exists in their universe, and also assuming they haven't developed zombie photosynthesis, logic would dictate that either they must eat, or they are powered by magic.
If they need to eat, how does a dead body process food?
If they need to eat and can process food, how do they store energy and expel waste products? Will a successful zombie become fat and leave a zombie poop trail? Will unsuccessful zombies become immobile and eventually die a second time?
If they are powered by magic, who is the magician, witch, sorcerer responsible for the spell; and does a counter-spell exist?