Daily roulette within Dungeons, Raids, Alliance Raids, PvP, (etc), repeatable Leves, optional sub-quests (if you’re that way inclined), Palace of the Dead (and its sister variation), Eureka, Bozja, Hunts – etc – not to mention every single job having their very own story-chain of quests. The list goes on.
MSQ is the main bulk and the only relevant one-time-only content. It is the easiest method of boosting a chosen job to max in one go, or you can spread it around multiple jobs if you wish as there’s an infinite source of daily XP for whatever else you may feel like playing.
There is no zone-specific content to speak of except for one-time sub-quests of no real import. The actual important stuff is either daily or repeatable. It is infinitely easier to level sub-jobs because merely having a higher level job provides a boost. You’ll also be part of the grand companies by the time you’re looking to level other jobs. If in a guild (FC), another boost. Not to mention the equivalent of heirlooms (for those with certain expansion purchases). Hell, even something as simple as just eating any type of food provides a flat +3% XP gain.
There isn’t much difference. I can’t heal a dungeon as a Warrior, just like I can’t tank a dungeon on my Shaman. I can level a Warrior & Shaman, or I can level a Tank & Healer in FFXIV (without the obscene gatekeeping or restrictions by comparison), with relative freedom to swap on a whim and maintain static progress.
I’m not saying one is better than the other, just pointing out that multi-role is not the same as multi-class. Saying that X class in WoW can play as a Tank, Healer or DPS does not translate to WoW having multiclassing.
A) You can’t do that as taking most types of quest as X job means the reward will be tied to X.
B) You don’t even need to. Far from it.
It actually gets easier to level alt-jobs due to the inherent systems, whereas WoW essentially bogs you down by doing so due to the restrictions associated with the individual aspects listed in my first post.