Having played a good few new MMOs the past few years I can't but feel absolute dread at the idea of coming back and having to level in WoW. Even a new expansion would make me cringe purely from thinking I'd have to go through the same old dated leveling system again. Overall I'd say that other recent MMOs have majorly spoiled people and that WoW's system really doesn't live up to standards anymore.
Do you think that the leveling system in WoW needs upgraded ? Especially with eyes on a new expansion I can't imagine anyone looking forward to yet another 5-10 hours / level through the same old system. WoW's leveling system to me can best be described as: "Kill x random boars, wolves, pirates, murlocs, monkeys, yetis, evil raiders, whatever in every new sub-zone you get sent to." It feels very disjointed and inconsistent, throwing you from one pointless little situation to the next. (Oh no, the catapults need fuel! Oh noes, some monkeys stole some crates! Oh noes, we need to gather some pine-apples for our soup! Oh noes, there are too many boars we must kill some! etc.)
In comparison lately we've been shown the freedom GW2 leveling allowed, it didn't try to force any specific one task down your throat. Or now recently the Neverwinter or Star Wars leveling system which felt more like a single player story or campaign mode, you leveled by going through a series of very focused storylines, not by having to kill 5 wildlife in every new subzone you got sent to. Best way to describe is by thinking of Half-Life and Bioshock and imagining that you'd have earned your 60 levels while playing through the storyline of those games. Or simply think of a single player RPG where the storyline is the main focus and you generally wouldn't find a series of mini-quests about random crap for every new subzone you'd come across. Imagine the Dragon Age single player storyline being a leveling experience in an MMO.
I feel that the leveling system they used up until now in MoP simply won't cut it anymore for a new expansion. People are used to much better by now, too many people already moaned how they hated the leveling from 85 to 90 in MoP already.
Edit: Leveling in MMOs originally started with "grind mobs until you level so you can move on to the next zone". WoW gave us quest-based leveling instead. But that is almost 10 years ago now... things have evolved beyond that point. The current standard of leveling in an mmo is at least at storyline-based leveling right now. And I feel that it doesn't do WoW any good to sticking to their old quest-based model. People who have been spoiled in other newer MMOs will have a hard time swallowing the old quest-based leveling model.