People like to feel their character is unique, and the ability to create your own build using abilities from various skill trees for each class fulfills that desire. With skill trees and proper balancing it is entirely possible for there to be a great number of valid builds for each class.

Every player knowing they are a carbon copy of others of the same class saps enthusiasm and causes a skillful playerbase to stagnate, leading to players doing just enough to not get kicked from parties. With builds, players have a creative outlet and a reason to contribute 100% in raids/dungeons/etc. by demonstrating what their personal build can do.

Gear sets: I'll keep this really short and simple: more than one endgame set of gear. The current model of "the highest iLevel is the best" needs to be re-examined.