First off for those of u that say 'watch the trailer, there's a sorta silent takedown in that', I mentioned in my OP that I saw that, if you didn't realise/read that that is what I was talkin bout then that's my mistake, I apoligise and should have made it clearer... Also Karl Fellhelm, your idea that it should be as a perk is very good, maybe sumpin like level 3 - 5 of that type of stealth perk, so that it takes up 1/10th (or close to it) of your perks... i.e. your're a stealth char... Perhaps there could be similar perks of this magnitude for Combat and Mage types as well...
EDIT:
I'm pretty sure the character will do a special execution animation only if the damage done will be enough to take down the enemy in one shot. Any attacks that fail to take down the enemy in one shot will not be given the special execution animation.
Good point.