I'm going to practice a little bit my english
I believe that what wow made a great job is in quest variety, thats why its fun to level up pj's in wow. So I believe that best option for leveling process is variety, for example, different type of quest like in pvp, pve, jumping puzzles, puzzles, gathering, crafting, world bosses, raids, achievements, party, grinding, farm raids, mini games, etc.