Got into beta and I have to agree whole-heartedly with the OP. Legion is shaping up to be a fantastic expansion. Questing feels great, leveling feels great, storytelling (not story) is fine, order halls are fine (and I was very skeptical of them), and there's actually a plethora of world content.
However, since Blizzard feels the need to reinvent the wheel every expansion, Legion, an otherwise decent expansion, is going to suffer.
Classes are in a horrible state. People might enjoy "class identity;" I do, too. Despite that, this thorough remaking of classes has left many bare and in a poor state overall.
The new talent system is partly responsible for this. The MoP talent trees were added in an effort to "offer players a choice." While I preferred the talent trees we had from PreBC-WotLK, the MoP system was fine. We're still using the MoP talent trees, but talents have been reshuffled, and talent swaps have been made tedious in an effort to make "talents meaningful."
This has created a few problems. The MoP talent trees made sense. Roughly speaking, the tiers were all grouped together (mobility tier, defensive tier, etc.). Now, we have nonsensical tiers that feature AoE, Single target active abilities, CDs and/or single target passives sharing the same talent tier. For active abilities, this makes keybinding a nightmare. I mentioned before that classes felt bare, they are; however, paradoxically Legion requires an absurd amount of keybinds. You won't be pressing a lot of abilities, of course; in fact, you'll only have less than half of your keybinds "in use" at any given time. I won't even get into the absurdity that is the new talent change system. They make talents very situational, take away themed tiers, and make switching talents tedious; it won't work well.
On the topic of talents, we've seen a major philosophy shift. Before Legion, talents were useful abilities that augmented ones class. Now, they don't augment, they "complete" classes. This is most obvious for fury warriors, the talents Carnage and Massacre (Both on the same tier now, horribly enough) should be baseline, as they make the spec as a whole work. Instead they're talent choices. While that's the most blatant example, there are more subtle examples on other specs, and it seems to signal a shift in design philosophy.
To go back to the lack of abilities that classes have, PvP talents are another culprit. I like where they're going, in theory, with PvP. Removing trinkets, gear, focusing on stat templates, etc. That's great (and long overdue)! However, while PvP talents are fine for PvP, they come at the expense of the rest of the game. I'll use warriors, again, as an example. Spell Reflect and Intercept are now PvP talents. For players that used those abilities usefully in PvE, well they're out of luck. In addition to the mistake that is pruning 2.0, the PvP talent trees have managed to remove quite a bit of class utility.
I expected the expansion after WoD would see class design fixed. I was wrong. Instead, everything else seems to have significantly improved, but it's all for naught, because class design itself has become much, much worse.
Class identity, talent synergy, meaningful talents and PvP talents sound great on the drawing board. PvP talents should've stayed there; everything else could work, but currently doesn't because of horrendous execution.