To clarify; in GW2, everyone involved in a fight gets full Exp and Loot from the kill, but in an 'event' such as killing a particularly tough enemy, the more you participate in it, the better your 'rank' and reward. (For example, a 'gold' rank for being in an event from start to finish gives you around 25% more experience, karma and coin than bronze)
Also; everything you do? gives you Experience. Exploring the world, and you discover an area? Hello, Exp. Find a new waypoint and activate it? Exp. Pass by a point of interest? Exp. Harvest ore or chop down a tree for crafting materials? Exp per whack of the pick or axe. Ressurecting someone who fell in battle? Exp.
Just by exploring the Four Cities in the beta (Divinity's reach, Lion's Arch, Hoelbrak, and the Black Citadel), I went from level 2 to level 7 without having to fight anything.
Also; at the end of the beta, my thief was level 16. to go from level 1 to level 2 takes 4000 exp. to go from 16 to 17 takes 6400 exp. The required experience per level is only a few hundred more than the previous, instead of a logarithmic scale as seen in older MMOs (The most notable offender being Lineage 2, where a single kill gave you around 3000 exp, and you needed 1.5 million to hit level 2, and 150 billion exp to go from level 79 to 80)