WoW story took a dive when it stopped being short tales of (mostly) little ultimate relevance and started trying to be a big, grandiose, overarching story. It just doesn't work very well, and even when the stories they're trying to tell are halfway decent, they're absolutely butchered by the restrictive delivery of the game. In a lot of ways, it's quite clear that they're not writing for the medium they've got, which is frustrating.
For me, the tipping point was Battlefield: Barrens. They tried to do something very heavily story-driven, but what that meant in practical terms was that I'd fly out of Orgrimmar, kill some Kor'kron in the Barrens, and then fly right back to Orgrimmar like nothing ever happened. There's a permissible level of flex between gameplay and story, and then there's a story that almost goes out of its way to be untenable for the sake of a plot that clearly hadn't mapped this out in advance.