I think DN was the first game I played where you walked about a real environment with realistic objects and furniture etc. instead of empty rooms on a spaceship. I remember seeing it at a friends house and being totally blown away that... he was running around outside on a street. It was also the first FPS game 'with attitude', Grand Theft Auto style. Also had some cool gameplay mechanics like CCTV terminals. In fact I think I only had a demo of it, not even the full game, and it still made an impact on me.