The horror of Romero's zombie movies is very simple--it's the horror of a sinking ship. You sit in a watertight compartment, and you wait. Water is a weak opponent, it can be kept out by something as simple as a finger in a leak. But it's patient. Sooner or later you let your guard down, and then it has its way with you.
Here's a tribute site to Day of the Dead. It's a funny website--it disses the movie up and down the block, then apologizes for its crticisms (I think its main problem is that it doesn't really make a case for the movie's greatness). The speculations on why the zombies resurrect are interesting tho. Imagine if Jesus were one--he'd be a kickass zombie.