Sufyan: I think the key elements that have never really been replicated in a single game since are as following:
- First person exploration and fighting using scarce resources in terrifying solitude
- Addictive character building system that keeps you up at night thinking of builds
- Richly crafted and presented game universe that is worth more than the sum of it's parts, the setting is not generic
- A well made machine antagonist that avoids the usual tedious 'self-aware AI' tropes (much rarer than you were probably thinking before I brought it up)
- Likewise, well made and interesting zombie/hive mind antagonists (first time I heard a monstrous creature cry "I'm sorry" as they took a swing at me is an unforgettable gaming moment)