I just killed the Ender Dragon for the first time in a new world. I died many times before I finally killed the dragon.  Every time I respawned and reentered the End, the dragon had full health. This occurred despite the fact that I had gotten the health below half on most occasions and all ender crystals had been destroyed. To make matters worse, I received no XP for defeating the dragon! See separate bug report on the dragon dropping no XP orbs.