Aside from what I posted earlier…
-They don’t back up or follow through with their own details and plotlines. Azerite’s the greatest example this expansion. It’s heralded as some super revolutionary material that everyone wants, and then proceeds to be fairly irrelevant for most of the expansion story. It basically devolves into being very good gunpowder and a kinda good metal. It’s not at all the game changing substance they tried playing it up to be. Also, from a player perspective, it’s everywhere and pretty much everyone is handing it off to you with no qualms or anything. So, even the value and rarity can’t really be established convincingly.
(Another example of things they can’t back up is Night Warrior. Tyrande and Nathanos facing off against each other was a complete joke. We also never see anything further from Night Warrior or the Darkshore Warfront.)
-Character motivation remains a longstanding issue. Baine basically does nothing until he decides Derek is the most awful thing ever. Saurfang is his own issue - choosing to sit in a cell and let the player and other NPC’s potentially die for his own honor, and then expecting the player to side with them. For Horde, siding with Jaina after she’s a raid boss is also problematic, especially if you play a Zandalari (who lore-wise never met Saurfang and has little reason to care about Baine).
-Another large problem is that it’s pretty much reenacting MoP’s faction war step by step, albeit just in a more ham-fisted manner.
-The biggest problem though is that they’re ultimately writing backwards and not providing sufficient cause or explanation for events. Basically, they want X to happen, so they get the pieces arranged for it, but they don’t really account for what was happening prior to X. So, things are just shifted around erratically. Look at the expansion intro. We spend a 2 week event on Teldrassil, then go defend Undercity, then go out to Silithus to talk to Magni, then go attack Stormwind, then head off to Zandalar. None of this is really connected with itself or anything else in the expansion. Us having the HoA is only brought up by Azshara - no one else mentions it at all in the expansion. We’re never given an explanation for why Talanji was in Stormwind or how Sylvanas knew this (but we had to be there to see Saurfang sulking). Lordaeron and the Forsaken themselves are never brought up again in any capacity. Teldrassil, Darkshore, and Tyrande / Malfurion are just an afterfront for a warfront. And, coming in 5.2.5 / 5.3, all of Zandalar and Talanji will likely be an afterthought. There will be no vengeance for her father, as everyone will be forced to unite against Sylvanas. That is why the writing is bad.