This one is for all of you:
I had maybe 4 minions on the board, facing two of his, and with ~17 life left. He was up around 25 or more. Druid. I'm playing Secret Mage, and I've got two 5/5 ethereals out, along with a mirror image KotG and a 2/3 Mana Wyrm.
Anyway, it's turn 9 and he does the Druid combo. Pops Force of Nature + Savage Roar. It's game over, right? Except...so I have a secret, and I guess it could be Ice Block. Anyway, he finds a way to get me to 1 life, and I figure he's just being safe in case of Ice Block. Then he rams his 7/5 Ancient of Lore...into an Ethereal. Turn over, I have one life.
So he was being a rude person.
Next turn, Ice Block, kill his only minion, go to face. Ethereals grow.
He's thinking whatevs, goes hero power to the face, trigger immunity. So then he drops the 4/6 taunt and a Spectral Knight.
I've got plenty of lethal on the ground now. Poly the taunt, freezebolt his face, hero power his face, then hit him with minions to finish him off.
That's why you don't leave opponents alive just to be mean.