There are quite a few elements that you may have overlooked while watching the trailer, especially if you watched it on Youtube. First of all, the beautiful textures. They’re extremely detailed and the high resolution normal maps make the grain stand out even more. It’s especially visible with clothes (just look at Aiden’s coat and sweater) and skin. The detail of tattoos is also another great indication.
The second element is the many fine details that are modeled in the game, and that are easy to miss. Facial hair is one of those. Even nasal hair is actually modeled strand by strand. A similar level of detail is visible in the grass of the tomb scene.
There’s simply a whole lot going on the screen, with elements that are often modeled in much lesser detail in open world games, but since they’re small elements they’re harder to notice. The elevated railway is a good example, with its extremely complex structure. There’s a whole lot of polygons that the engine has to move there.
Another element is the draw distance. It’s extremely high compared to most open world games. There’s no distant fog to keep the polygon count low, and it definitely makes a big difference in creating the sensation that we’re walking in a real city and not into a series of blocks where you can clearly see only a few hundreds of metres away. The whole city is visible, and there are thousands of objects between buildings, vegetation and vehicles. And Watch_Dog doesn’t seem to use many of the usual tricks that keep things lighter on the machine, but degrade visibility radically at a distance.