Find any video from Wizardy Online talking about cross-classing and Elite classes and it will immediately become obvious why the class-Job system is bad. Final Fantasy I's class-job upgrade was lifted directly from Wizardy's Elite classes, just using different names. Where as in Wizardy the elite classes were 50% one and 50% the other class, in FFI you just got additional skills in the same class, and it wasn't half of anything.
From a design concept, you can pull 10 skills from any/all classes, some of these are useful, many are not. When you upgrade to Job, you get restricted to 5 skills, and only "useful" skills.
So the upcoming Role system does away with being able to cross-class anything from another role, but it's ultimately the same thing, just now you can't have Tanks with Healer skills, or DPS with tank skills. To simplify things further they removed skills that overlapped too much to reduce cross-bar creep.
To really look at how the Job system in FFXIV really works, you have to go back to FFV. You initially start in "Freelancer" and can switch to any job without changing your base level (that's how FFXIV V 1.0 worked) That base level was stripped out and now you can only switch between jobs you have main hand weapons for. It's an improvement but also screws up the job-progression since you can't fork off a class into two different jobs of a different role, as one of those jobs will be "half and half" (see my wizardy statement above)
So ultimately the intent was something that MMORPG's were trying (or in some cases had tried already and wound up switching to a role system too), but didn't actually do well. Historically even in single-player RPG games, being able to arbitrarily change the class of the character leads to a kind of power creep where one specific configuration leads to nothing but face-roll gameplay.
Could you imagine if you could just cast X-zone/Death on every monster and it would be over like in FF5 and FF6?
Players want to be outraged. As an equal comparison in the game Mabinogoi, up to a certain point, the game had no actual classes, you could learn any skill arbitrarily, and if you screwed up you "rebirth" back to level 1 and level other skills, repeat every week. Then Nexon decided to introduce actual classes into the game and there was extreme rage and in-game protests in Korea and in North America about "stuff being taken away" like not even close to the outrage with WHM here is. Nexon relented a little and didn't put in any of the debuffs, but you only get bonuses if you pick a class, which means any class you pick gets double training points, but if you use skills outside that class, regular "freelance" training rate. This feature otherwise would have made it so "healer" players unable to do any content alone.
People actually preferred that "min-max BiS" freelance mechanic and didn't want to become yet-another-cookie-cutter Fantasy WoW game. Classes means they actually have to pick sensible classes for content, and since new content skills often had the same progression as old ones, it was very possible to have every skill in the game, and still suck at hard content because because you don't know which skills are useful in certain content, and newer content often required using the new skills, and if you use the new skills on "hard mode" versions of old content, it was still a face roll. Like people made a point of solo'ing all old content using no skills at all because the base strength (no level sync) ensured there was no challenge.
FFXIV's best features is the level sync mechanic, because it ensures you can't just spam a dungeon in 30 seconds by running straight to the boss room and flicking the bosses nose and having them die. You can't be carried by having one over-powered player just blowing through the content.
One of FFXIV's worst features is the cross-class system, because it does not enforce picking useful skills for your role, rather it discourages teamwork by having every one pick the BiS skill, and ignoring everything else. The Role system, in theory means the role skills can be divided between the players playing that content in 8/24 man content, where as before everyone just picked the same thing.
As much as I hate being "forced to play a role", I would rather that be enforced during party play to encourage teamwork rather than everyone-for-themselves style play that many other MMORPG's evolve into, because the content becomes too easy too quickly. If Square-Enix is guilty of anything, it's allowing the content to become easy by not adapting the content to the player's power-creep as part of level-sync. If there is a 20% differential between wearing level 50 gear and level 60 gear in a dungeon that is level synced to 50, then (4) players are at least 80% too powerful once level sync kicks in, and in 8-man content, 160% too powerful. So what will happen with level 70 gear? Will players be able to just solo level 50 content while synced? I think that needs to be addressed.