It's that they're locking more class fantasy behind swappable player power. Dark rangers are fucking awesome, but there's inevitably going to be a M+ fight where you need to suck it up and swap to Sentinel even if you hate them and your character is undead.
You're inevitably going to have to choose between the fantasy you like and the optimal build for your spec. That's just definitevely less fun than being able to pick the fantasy you want. These should either have minimal gameplay effects, like racials, and be locked permanently as a fundamental part of your character like race and class.... or they should just be 100% cosmetic.