I just wrote some of this in another thread but this was mostly what happened with ShB's overhaul. Much of it was praised for the substantial QoL improvements.

1) Removal of subclasses and the need to level other classes for necessary "cross-class" skills. As example, to become a Paladin, Gladiators needed level 15 in Conjurer as well as the 30 in Gladiator. Paladin could use Cure and Raise. Flash and Provoke were specifically Gladiator skills, so effective Warriors were expected to level Gladiator enough to gain these skills. This type of thing was littered across the board with some jobs requiring much more initial work to function at high levels.
2) The addition of the role skills and trimming of other extraneous skills. This fully replaced the above system. Now you simply gain those necessary utility skills as you level up normally, but at the cost of a lot of options for cross-class skills. This also required a lot of internal balancing. Spells like Stoneskin and Protect were deleted and aggro management was retooled to allow DPS to not worry about stacking aggro reducing skills since they simply didn't have them anymore.
3) Removal of "bonus" customizable stats linked to class level up. At certain level thresholds, you would gain a stat point you could freely put into any stat you want. No one "customized" as it was too efficient to put everything in your job's primary stat. This very much hurt if you liked playing both Summoner and Scholar as their stats are both linked to Arcanist. So you had to pick mind for Scholar and nerf Summoner, vice versa, nerf both by doing half and half in mind and intelligence, or keep buying the expensive item that reset said bunus stats.
4) The addition of job gauges. This made it much easier to parse job specific information and added a lot of beloved job identity. It was really rough for some jobs initially, however, since a few just didn't really need a gauge or had a new gimmick tacked on.
5) Retooled Phys.Ranged to remove the cast bar style introduced in Heavensward. Everyone hated it and it was a DPS loss. No, really, there was much rejoicing... For Bards. Machinist has never been entirely well. It's my favorite job.