Fallout is all about player choice and consequence for me; it's an idea of allowing the player to make decisions in an open world, in terms of gameplay systems, story and imagined role, and to see the consequences of those choices. You don't have to be a Fallout game to do this, but I've always felt Fallout handled it in a distinct style that pushed combat to the back of the focus and allowed for a lot of very legitimate, alternate play styles. That said, essential to the Fallout experience, for me, is the post-apocalyptic theme. I'm not so much opposed to civilisation in the games as I am to a modern-day world. The idea of a society built on the bones of another far more successful one is fascinating to me, and in addition to providing a familiar-yet-different universe to live out a character, Fallout's setting allows for commentary on human nature, political organisation, human rights, the value of morality and ethics, etc. Beyond merely allowing for said commentaries, those commentaries are actually essential and integral to the Fallout world at a fundamental level, and that's something I don't think many other games can do.