And doesn't your opponent have to be completely helpless before you deliver a Coup de Grace?
Ah yes, after looking in the PHB, Coup de Grace, and subsequently, Death Blow, only works on a helpless defender.
Helpless defenders are, "Someone who is bound, sleeping, paralyzed, unconscious, or otherwise at your mercy."
So just being stunned, or nauseated, panicked, etc., doesn't count.
To help further the "helpless" definition, you're only helpless when you would effectively take a -5 penalty to AC due to your Dexterity being 0, plus the +4 AC a person would get since you're not moving at all.
Coup de Grace automatically hits, but always provokes attacks of opportunity.
You may only make Two Standard Actions per-round. So, if Death Blow must be a standard action, no character could ever make more than two-attacks per round if they were using the Death Blow combo.
Anyway... i can see this combo working well for a monk character. Take "Freeze the Lifeblood" at higher level, paralyze your opponent in a standard action, and follow through with your auto-crit and see if you kill that sucker.

And... since you do high damage... And... since you'd get 1d4 rounds of it... yeah, any humanoid that got caught in Freeze the Lifeblood is essentially dead.
I'll have to try that combo in the next Eberron campaign I might get a chance to play.