Alliance had more zones,and with it more quests. They were more streamlined together and you did not have to grind out as many levels as there were always areas to do said quests. The Horde was screwed in the barrens and Stonetalon (the only places I vividly remember being forced to grind out levels because there were no quests to do, there were others.) Cata made it so the amount of zones were much closer to being equal, as well as neither side would no longer run out of quests and be forced to kill mobs to level enough for the next zone.
Basically leveling fucking sucked for the Horde in classic and up to cata. with shitty run paths to get to said quests and not nearly enough quests to level on them alone.