Experience can be gained by killing mobs, completing quests, and exploring the world. There is only so much of the world to see, and quests generally require some spilling of blood, so experience points are most regularly gained by killing mobs. That is, when the mob dies. Because of this, I was taken quite by surprise when I gained 66th level by charging in to battle, the fireworks surrounding me before I even hit the mob.
I was only a few thousand XP away from 66th level and flying up to Light's Hope Chapel to hand in a world-event quest from the Argent Dawn, thinking that the XP gain would push me over the boundary to the next level. To my disappointment there was no XP gain for the mission, so I remained a few thousand XP shy. Hoping to defeat the Headless Horseman—optimistically, it turned out—and thus needing to be in the general area I was loath to head back to the Outlands merely to stomp on a few critters. Being in an Azerothian level-cap zone I instead wandered around looking for something still considered dangerous enough to offer me XP on its death. Luckily, the high-level undead that infest some of the hills still conned green to me and I started killing them for sport.
What I had forgotten was that I had not quested in the Eastern Plaguelands at all, and I think I only got the flight point because a quest from Winterspring brought me here, so my map was almost entirely unexplored. I was killing my way up the hill when, in a wonderful coincidence, my charge in to the next mob crossed me over the boundary to discover a new area on the zone map. I earned another discovery ding! It was a little odd to level up entering battle, though.