1. Simple but immensely replayable software. It's simply genius how everything works together. This game never gets old.
2. Simple but immensely replayable playfield. It just rules. Smooth, great flow, every shot is satisfying and usually sets up another shot. Plenty to do with either flipper.
3. Awesome art, theme, and humor (it's actually funny unlike MM).
4. It's created an instant litmus test to separate good pinball players from bad ones. If they say "I hate that game it's just shooting up the middle" they are bad players.
5. Awesome sound, dots, voice work.
It's the most replayable game, ever, and no matter how well you do you you always think you can do better the next time. You want to hit start even after hanging 50 billion on it, unlike so many other games were long games are brutal chores that leave you never wanting to play it again.
It's the best game ever made and that's not going to change anytime soon.