One of the ideas most crucial to the original Deus Ex was that it allowed players to approach a wide variety of scenarios and complete them using the toolset given to them by their skills, augmentations and abilities - sometimes in ways that the game's developers never anticipated (including occasional exploits of the game's scripting and AI).  While part of this game out of the fact that the game was, understandably, less technically sound and open to certain forms of abuse by dedicated players, in almost every situation, Ion Storm went to great lengths in order to ensure players always had reasonable, logical options available regardless of gameplay style.
 
For the most part, this philosophy has been retained in Deus Ex: Human Revolution.  Objectives during missions often feature multiple stages and allow for many options and outcomes.  The fortitude of the game in this respect is extremely impressive.  Early in the game, for example, the player must negotiate with a terrorist leader who has taken a hostage.  While the game's options explicitly present the options of negotiating the release of the hostage, allowing the terrorist leader to leave with the hostage, and simply entering into combat for a lethal or non-lethal takedown (which normally results in the hostage's death), there is some additional nuance in the decision that the game actively recognizes.  During my third play-through, rather than negotiate or allow the terrorist to escape, I tried something else: I fired a tranquilizer dart at him immediately upon drawing my weapon.  Rather than watch the hostage die as the temporarily-invulnerable terrorist executed her, as I'd expect in so many other games, instead he simply fell over unconscious.  Not only did the hostage live, but so did the terrorist; later, I encountered him held in prison, and even later in the game, he ambushed me after escaping the prison.
 
This sort of detail is a subtle one, but it adds an incredible amount of meaning to the player's experience.  The outcome I received was not one that the game telegraphed, and it was not one I expected to be possible - and yet it was, due to my quick reflexes.  In a sense, I felt as if I "broke" that scene, and Human Revolution simply shrugged and said "yeah, you can do that".  In many ways, this situation parallels the famous moment in the original Deus Ex, where the third unstated option of killing an ally in what is presented as a binary choice leads not only to the player's surprise at the success of the act, but the characters in the game itself seem to be equally surprised, even as they desperately come up with ways to cover up the player's actions.
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