Cautious, a Warrior, leveled to 80 without dying.
She had no deaths. None. Nada. Zilch.
There is no achievement for doing this, however I wouldn't blame Blizzard one bit if they went in and retroactively added one just for Cautious. Lord of the Rings Online has a no-death achievement, but only up through level 20. I got to level 18 without dying on LotRO on a recent attempt, however I perished when some "really really super awesome person" trained a bunch of mobs near me just as I executed an AoE attack. "Really really super awesome person" is what I called him too. Honest.
A few months ago in The Queue a reader asked if there would ever be (or is) such an achievement for WoW, and my response at the time was no, there is not one now and probably would not be one later. I reasoned that including such an achievement would just present undue psychological damage on the player if they died at 79 from an unforeseen circumstance. I still don't think there will be one, but it's pretty awesome someone managed to do it sans in-game achievement.
Some of her key stats upon reaching level 80 include: