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:
Quests completed: 1,777
Healing Potions consumed: 19
Hours played: 296
To me this indicates that the majority of the time was spent killing mobs and completing quests that were lower than her level. Nothing is wrong with that at all, especially if trying for such an achievement. The 296 hours played is 12 days 8 hours, which isn't too bad considering how slow and careful she was going.
Players will ask if this is the first person to reach level 80 without dying. It's entirely possible Cautious holds that honor, but we can never be sure without Blizzard confirmation.
It should also be noted that the armory reflects one death, that occurred sometime within the last 24 hours since we got this tip. That death happened after reaching level 80. Cautious posted in the forums that this happened via use of Nitro Boost.
Edit: Updated how the only death after 80 happened.