A short video comparing the visuals of the two games. As you can see, Bungie added lots of light sources in indoor locations that had previously been very dark. Between the two games much of the abandoned wreckage in the skybox in the distance was plundered by the Fallen. And a few locations meant to be inaccessible were pushed further away from the play space, probably because Bungie wanted players to stop jumping off the map to try to get there.