Pet Sematary is one of my all-time favorite horror films for many reasons. First of all both the novel and the screenplay were written by Stephen King. He is simply the master of setting an ominous mood and cranking it up to full force by the end of the story. If someone else had adapted his novel, Pet Sematary would have definitely been a lesser film.
Secondly, director Mary Lambert, who did a number of award winning music videos for Madonna, has an impeccable visual style and each scene is lovingly composed. There aren't too many women with an interest in doing horror and Pet Sematary is all the proof we need that there should be more of them.
Thirdly, the special effects are mostly subtle but really good. I especially LOVE the work they did on Brad Greenquist who plays the ghost who keeps reappearing all throughout the movie. The other outstanding creation is Selda, the sister of the main characters who was actually played by a man, Andrew Hubatsek. If that doesn't creep you out, I don't know what will.
Finally, we have a dream cast anyone would love to work with. Both Dale Midkiff [The Crow: Salvation] and Denise Crosby [Star Trek: The Next Generation] are perfect together as Louis and Rachel Creed. They help to create a family we really care about and that's essential for this film to work. The other standout is my beloved Fred Gwynne [The Munsters, My Cousin Vinny] who plays the next door neighbor, Jud Crandall. I think he is perfect in this role and shows he was just as great at drama as he was at comedy.
What more needs to be said? If you've never seem this film, what are you waiting for? I count Pet Seminary as one of the best Stephen King adaptations out there. I never get tired of watching this film.
For more info check out the film's entry in IMDB.