well actually that is the HD graphics, not only is it rendering the game at a higher resolution but the textures have been made larger.
look at the text on the signs.
You can make it out better in the HD version. also some props [objects] and the characters have been remade with more details [polygons].
it also seems like they have tweaked the FOV of the camera
http://en.wikipedia.org/wiki/Field_of_view
http://en.wikipedia.org/wiki/Focal_length
as you can see distance in game is better represented. i.e things seem further away and less flat. in photography terms that's the focal length
