As a first person 3D game, what are the most defining, inherent aspects of the game?

First person 3D games are fundamentally concrete, so all of these aspects are concrete.

1. Player location
2. Player facing
3. What is and isn't visible to the player
4. Spatial relationship between the player and other objects

Features such as player health are more abstract and are injected into first person games. (That is partially why health is usually displayed in an abstract HUD instead of within the 3D environment.)