The Power of Choice
I must admit that this is the first and only game I've played in the Deus Ex franchise. Therefore, some of the comments I make may be common knowledge to diehard franchise fans.
I love stealth games. Since most titles are geared towards engaging in some form of conflict, there's something very refreshing about an experience where part of the gameplay is avoiding confrontation. As you sneak around the various environments, there's no denying that the atmosphere in this title is absolutely fantastic. From the background score to the detailed settings, future Detroit feels like a living, breathing city. One of the main contributors to this immersive experience, and what I think is hands down the most enjoyable aspect of this game, is the multitude of options available to complete every mission objective.
Since the gameplay mechanics hinge greatly on how players choose to level up via their praxis points, the simple act of gaining access to a building completely changes depending on in-game decisions. By affording players a sense control, the designers of Deus Ex: Human Revolution have given players the coveted feeling of influence. While I wouldn't exactly classify this title as a sandbox game, it definitely feels that way when you stop and consider the what ifs.
If I had invested in one more praxis point to increase my character's arm strength then I could move that box to get access to the floor vent behind it. But I didn't, because I decided it was more important to have optimal hacking skills. Oh look, a password protected system that when accessed will let me unlock the door that leads to the room that floor vent is connected to. Whoooooah.
Despite my comically simple example, this design is extremely intelligent because it gives players direct feedback on how the choices they make effect the flow of the game. While in my example, the same goal could be achieved by different means, I'm sure that there are secrete spaces I was never able to uncover because of how I decided to level my character. In short, by making sure that players can't do everything, they ironically feel like they have more control over their experience.