I think what the various abilities dealing damage actually does can be very abstract, especially considering the "fluff" and "crunch" related to abilities in 4E can be quite separated. Thus I think an alternative way to the normal rules could be that each X damage overkill (damage past 0 HP) increases the chance that the enemy is killed instead of unconscious, and you can probably weave in some houserules about decreasing the chance of inflicting lethal strikes by taking a damage/to-hit penalty to attacks.
If the players agree with it, you can work through the powers/characters to determine who have the most "lethal-risk" powers and tendencies in combat. To throw a bone to whoever end up with "very high lethal chance", maybe grant a bonus effect for killing targets with much overkill (like making enemies prone to panicking take penalties when the overkilling guy is around due to them being scared out of their pants, etc.) That way there's kinda an upside to both playstyles.
If the players agree with it, you can work through the powers/characters to determine who have the most "lethal-risk" powers and tendencies in combat. To throw a bone to whoever end up with "very high lethal chance", maybe grant a bonus effect for killing targets with much overkill (like making enemies prone to panicking take penalties when the overkilling guy is around due to them being scared out of their pants, etc.) That way there's kinda an upside to both playstyles.