A line from the assassin design diary caught my eye:
"With D&D combat, the tension is highest at the beginning of the fight (when players and monsters have a full array of resources) and lowest near the end (when players are out of resources and monsters are dwindling)
I think that the wizards designers are working from a false premise. Now I like the assassins power mentioned in the original post and in general I'm fine with powers that don't kick in until later in a fight (especially for monsters), but I disagree that tension is highest at the start of combat because all combatants have full resources.
The start of combat is when a monster is best able to withstand a daily or encounter. A few rounds in after hammering a foe with daily/encounter they are often then put down by an "at will" and "at wills" are no longer the 3.x and prior boring "I swing my weapon doing damage X"
Tension should be higher as the combat progresses, resources dwindle and the outcome hopefully balanced on a knife edge Tactics such as one poster mentioned above of bringing in reinforcements is a great tension increase. Tactics of the monsters should also be in great play to bring tension to the battle. To paraphrase a recent Governor Ed Rendell quote "let's not wussify the players" there don't need to be additional rule mechanics to increase combat tension....players can manage their own resources if they want to save a big hit and starting with monster choices DMs have plenty of existing tools to create and run tension filled combats.