Avalanche deserves an applause for the level of details in their games. Generation Zero and both theHunter games are fantastic. I remember first playing theHunter some 10 years ago, and was just stunned by the detailed landscape. I am stunned in the same way now, with GZ and COTW. I dare say no open-world game can match it!
Also, I have never played a game as atmospheric as Generation Zero. The details and the graphics play a big part in this, along with the music.
Walking in heavy rain and thunder actually makes you feel the need for a roof over your head!
Seeing the sun rise and set trough the falling leaves makes you want to just sit down and watch.
Walking through the woodlands in heavy snowfall at nighttime is jaw droppingly beautiful!
The postbox racks and the roadside sandboxes look just like the ones we have or had here in Norway,
and brings back some childhood memories. 
I love the patriotic touch that all cars are either a Volvo or a Saab! 
A few suggestions towards perfection:
When holding a scoped rifle, you should not be able to see the crosshair on the back of the scope.
More variation in house interiors. Not important, but maybe it will be worked over some day?
The game should be set in 1984 or something. If it begins on november 14, 1989, the 80’s are almost over. You play for a few days and think: “Hmm… It’s actually the 90’s now.” At least I do, haha! 