Graphically, the biggest impact of the game, and also the biggest surprise for me personally, has been the streams, which are amazing. The rivers and waterfalls are pretty cool as well, but I had seen those in preview videos. Also, clouds in this title - they way they sort hang from the siides of mountains - are also very well done.
I have to tip my hat to Bethesda for their work here in making me feel like I'm adventuring in an elevated land. Even when I'm down low in the plains I somehow have the sensation that I'm already high up in the world.
Any DLC coming? More streams please!