A 40-damage coup de grace would instantly kill a creature with 80 hp. If you deal the "bloodied" amount of damage in a coup de grace you slay the target outright according to the PHB.
My 1st level group had a little bit of frustration with the coup de grace rules during our game this week. The wizard put a group of goblins to sleep and the paladin was going to coup de grace a goblin warrior (lvl 1, 29hp). His max damage on his at-will power was 14, which was one point short of dealing the bloodied value of the goblin warrior and slaying it.
I thought about what to do... on the one hand, it is perfectly realistic in my mind that it might take two blows to kill even a helpless opponent. You might mortally wound them with the first blow, but they might be twitching until you nail them again. Hitchcock used to enjoy stressing how much of a gruesome and horrifying thing it is to actually try and kill a person. Also, the paladin was not at all maximized for damage and was in the midst of a melee attacking an opponent wearing some measure of armor... It was not difficult to imagine the goblin surviving the blow.
On the other hand, it was only one point off and I wasn't really going for a gruesome or horrifying tone for the game, rather I was aiming for epic heroism (although it could be argued that there is nothing heroic about a freaking paladin taking the time to coup de grace a helpless goblin when there were standing foes in melee range).
Since it was a trial run of the new system, we discussed the new rules for coup de grace around the table for a bit. One guy thought that the rules didn't work if the pally couldn't kill a 1st level goblin with a coup de grace. I agree with an above poster that it is situational and the rules are a great guideline.
The paladin decided that he could kill the goblin if he used his divine strength(encounter-class feature) to raise his strength before the strike, so I decided that he could kill the goblin if he used it, and we moved on with the fight. Any encounter power would have finished off the goblin. Any striker class could have killed the goblin with an at-will power. A more damage-maximized paladin could have slain the goblin with an at-will power. I think it works the way it is.