Infinity Ward made a tactical error in Call of Duty: Modern Warfare 2 when it included a multiplayer perk called Commando that increased the lunge distance for one-hit-kill melee attacks and, in its Pro form, eliminated any damage taken from long falls. This was not a universally loved feature, especially among those who frequently fell victim to knife-focused custom classes (which is to say, all of us). For that reason, it won't be returning in Modern Warfare 3.
Robert Bowling, IW's creative strategist, confirmed the detail in a weekend tweet. He also hinted at a more "balanced" melee attack system, though that may simply be further reference to the removal of the decidedly unbalancing Commando perk.