Console Monster writes: "Right, before I start I must say you need to play Execution before reading on. The entire experience is derived from not knowing anything about the game. DO NOT READ ON UNTIL YOU HAVE PLAYED THE GAME TWICE! (which you can do so here)
Ok, played it? Good
Now, it's not an entirely original idea. Far Cry 2 sort of did it by having your Buddies die permanently if you somehow managed to mess things up on a mission, and their services are no longer available to you thus detracting from the experience. Hideo Kojima took the whole "face your actions" route in Metal Gear Solid 4 later in the game (you'll have to play and discover it for yourself). But Execution takes the idea of making you face your actions and see the consequences to a whole new level.
The game can take as long as you want to finish. You can complete it in a matter of seconds or over a few hours depending on how you look at the situation. So what did you do? Did you shoot the person straight away? Wait a while, then kill him? or look for a way to not kill him? Whatever path you took is a sort of look into your inner gaming psyche, and it reveals quite a lot about how you game. I shot the person after a few seconds of deliberation, but saw no way out of shooting the person so I just shot them. That could mean, I like to look for alternatives to killing (which is true in the case of Mirror's Edge where I ran more than I fought) or I just got bored and shot him..."