What made the film so scary...
I think I figured it out. Okay, I am not a religious person but at the time of seeing the film as a young teen I was probably closer to an agnostic and still frightened of the possibility of going to Hell. The reason why the film was terrifying for me (and possibly terrifying for others the same reason) was because the events unfolding were legitimately the apocalypse. In this case this was the particular order it happened... the dead over running the earth.
If you pay attention to the clues in the film, some of the statements made and other events as well as some of the character's (pay attention to who ''lives'' and who ''dies'' - you'll start seeing a pattern) it's pretty evident that the Zombie's came from Hell. This a reckoning on man by God (let's just pretend he exists for the sake of the film) and it was entirely the end.
Other Zombie films and TV series never come close to being as scary as this film not just because of their failure to produce this films ominous atmosphere but because the other films feature the cause being an accidental outbreak of a virus 90% of the time. And the other ten percent of the time the cause is unknown. In this film however it's strongly hinted that this is a wrath from God and man kind is done.