I think one essential part of the Ultima formula was its geometric discreteness- The world was one in which the physics is broken into motion in orthogonal directions through discrete cells, and everything in the game leveraged this fact for maximum effect. It led to a really different gameplay feel than many modern games. Other games of that era had that as well, of course, but not many really leveraged it as well (Boulderdash and Archon would be two other highlights)

Some modern games of course also use this approach to physics (Advance Wars comes to mind) but it is far less common.