Extremely underrated game which has a simply beautiful take on post-apocalyptic environments and a very satisfying shooter mechanic. The progression / skill tree, though it has menus within menus in an odd fashion, is always rewarding and satisfying to explore when you get a new gun or ability.
I found the open structure fun to explore, even though the story only progresses one way. It is not an RPG after all. As others have said, it is very addictive. You always want to get to that next map marker. That being said, when life calls, the game is chunked into distinct encounters and areas, so it feels good to complete that bandit base and quit.
The game moves "realistically" slow on nightmare and makes you think about what you are doing before charging in. When you achieve mastery and are moving fast through enemies and unleashing hell - it really is a thrill because one slip up can wreck the whole plan, even though you have a large variety of weapons / abilities. I got this on sale recently on a 3rd party "game keys" type site but I really do love Bethesda's games and feel guilty.
Guys, I'll buy the next game day 1, promise!