Skills being tied to gems instead of classes is one of the main things that turns me off from PoE. I know that some people like it that way, but it is not for me. It takes away too much class identity and for me personally it is too ‘sandbox-y’.
The other side of the coin is also less class identity and less interesting itemization.
I think it is fine if you have items that proc Skills on Hit / on Attack / on Kill / when you Get Hit / etc, and also too have oSkills that give e.g. +x to Fireball (as long as this Fireball has only e.g. 3 Charges), but a PoE-style approach to procs and Skills/oSkills goes way too far.
For example procing Tornados on Hit on a Windforce Bow (Legendary) or a Thunderstorm on a “Legendary Thunder Hammer” perfectly fits thematically (even if it are just a “Flavor Affixes”), or giving oSkills on some Legendaries (see images below) is fine since then you have to sacrifice an equipment slot for making a build around that…
https://imgur.com/YWUAoWD
https://imgur.com/PwJuB0F
https://imgur.com/LvFTqF0
… but giving every class access to all skills is way too sandbox-y (for my personal taste) and also would not feel like Diablo imo, and on top of that would also decrease class identity.