In my opinion the parts are 
**
-the size -> 256 km2**
-dynamic light and gi
-the high quality meshes (e.g the cliffs,…) -> complex shaders, high res textures,…
-AO
-soft shadows
Btw, make sure to check out some other videos like the UE4 trailer >(e.g the scene at 0:40) or the twich stream -> there you can really see how good it looks like (in my opinion crysis is nothing against demo -> but kingdom come deliverance would be a good example for an open world project made with the cryeninge, but there the grass render distance is also pretty low :p)
@, is there probably a way that we can get some high resolution screenshots? 
