IMO the thing that truly make MI unique are the fans. Go to any adventure game discussion, and MI will sooner or later come out in some form or references. It was the yardstick of adventure gaming in the heydays, and the fans still hold the game very close to heart.
MI is not even cultish like Sam & Max, Space Quest or Zak McKraken. It is mainstream with lots of fans, who could still recall fondly of the previous installations.