LostSoul
Adventurer
EDIT: Re Lost Soul's comments about binary win/lose: In earlier editions, the attrition model was a built-in check against binary win/lose battle conditions. So long as attrition was possible, even a relatively simple battle could have important consequences later. Thus, each round of a battle, even if you were certain that you were going to win could be important, thereby sustaining tension.
Healing surges, daily powers, and action points do this in 4e. Attrition is still there, it's just changed so it affects every PC and not just the spellcasters. (Hit points in 3.x were not part of this attrition model.)
Do they add enough tension? No, I don't think so. That's why I think the goal of an encounter shouldn't simply be "kill all the monsters." "Kill all the monsters before they sound the alarm" is cool.
On the attrition model in earlier editions: I found it annoying. It was cool if you were a Fighter trying to watch your hit points. It was annoying if you were a Wizard wondering if you should use your spells or not, and then resting with all your coolest spells uncast because you were never put in the situation where you absolutely had to cast them. Walking around with one arm tied behind your back = annoying.