Im agree with Jastro. Having different problems in every starting zone having to do a "campaign" would be really good, like in wow is done, but please, don't put quests like: "kill 10 wolves" "bring me 20 teeth" "talk with my wife oh my wife is dead kill 4000 pigs". That's plain quests that every mmo have and i'm personally tired of that. I think it's better if it's always about the mainly questline. Example below:

Worgen, goblin, deathknight chain quests were BEAUTIFUL (at least, for me) but orc, troll (until cataclysm), human (in general), belf... were really boring. I think its need to put a general problem to deal with during 15-20 chain quests. Like i said: worgen chain-quests were really worth for me.

Moreover, Draenor introduction was a masterpiece, showing you firstly the main problem, after it the kind of enemies are you dealing with in the near future (like Gul'dan, Cho'gall...) and the chaos you made using the "worldbreaker weapon" from iron horde, every npc you saved and followed you put you into the context of what are you finding at the very beginning (garrison followers). I don't know if you are agree with it, but i think that bringing important npcs (like the introduction for new chars in Shadowlands that are with you in the first quest... into Pandaria too, Aysa and Jin and the Master Chang Xi...) shows you that it's "important".

I'm adding something: choice questlines. Variable reputations, special quests... into main questline would change the future (even mainly questline) of the whole game. The future of factions if you decide (% players) to help X and let die Y. I'm always using the same example: Defias and the Alliance. Why are u forced to kill every Defias u see? Bloodsail Bucaneers reputation could be farmed killing goblins, but sincerelly, is it the only thing that a mmo can offer you to enjoy leveling reputation? Spamming kills or dailies? I think crafting jobs, pvp and pve quests would show something new. It would be a thing into starting zone quests, knowing who are ur truly "enemy" and putting you into "good, neutral or evil" state and depending of yours, get helpful buffs, trading offers... with the faction u wish. Obviosully it's not a good idea if gameplay lore is being like wow, where everybody is Alliance - Horde.

Personally i prefer to get questlines like those and don't deal with 8548584385356456 random quests, so i think it'll be better if there aren't 49324384 quests in every place, its better for developers and for players. Leveling with crafting, minimum grinding (for getting some special stuff), dungeons, pvp... would be my favourite way to get lvl 50 and don't deal with boring and generic things.

Then, i would like to got 20 - 25 questlines in starting zones that shows you the lore of your race, a general problem that's getting solved at the end (or not). And a soloable dung (1 - 2 bosses) getting your first blues (for example). 

Thanks :)