BfA's story isn't disappointing because it's bad. On the contrary, it's not bad at all. It's just painfully mediocre but with just enough glimpses of a great story that we could've gotten thrown into the mix to frustrate lore maniacs. Blizzard's primary fuck up is that they made it seem like it would go in a particular direction and then fucked it up and went in an entirely different but much more boring direction. Here are some examples:
Expectation: BfA will be a story full of moral ambiguity and political intrigue.
Supporting evidence: Legion and BtS set up a nuanced conflict in that the Alliance fired the first shot and Anduin turned out to be not as experienced and masterful at handling the diverse factions of the Alliance as his father was. Il'gynoth's whispers had people believing that Anduin would make mistakes that would cost the Alliance and Azeroth at large dearly.
Subverted: BfA's conflict is as straightforward as can be. There's no moral ambiguity: Sylvanas' supporters are entirely evil and Saurfang's Rebellion and the Alliance are paragons of moral goodness. The fact that Genn fired the first shot is never brought up in a meaningful way. Anduin's inexperience is never a hindrance to the Alliance; he never has to face the wrath of the Night Elves for failing them nor peasant uprisings for drafting farmers into the army. Even Varian had to deal with the consequences of Northrend Campaign when the farmers in Westfall rebelled. But not Anduin. Anduin can do no wrong.
Expectation: there will be a naval conflict.
Supporting evidence: Ogmot prophecies of "two great armadas clashing upon a sea of blood". The first stage of BfA revolves entirely around rallying Zandalari and Kul Tiran fleets.
Subverted: the only major naval battle between the Alliance and the Horde happens at the climax of the Battle for Dazar'alor but it's not really that interesting. The fleets get taken care of in a one-off cinematic and no glorious naval battles happen.
Expectation: the war in BfA will have a beginning, a middle part and a climax.
Supporting evidence: it's common sense that stories should follow a certain arc.
Subverted: the war has a strong beginning, and... Nothing else, really. The Horde never manages to retaliate after Dazar'alor and the war itself ends very suddenly and in the most anti-climactic way possible. The differences between the Alliance and the Horde don't get resolved in a meaningful way either, setting up a MoP v3 a few years down the line.
Expectation: N'Zoth's return will be glorious and earth-shattering.
Supporting evidence: all the prophecies. The faceless ones rambling about Ny'alotha rising from the depths.
Subverted: N'Zoth's return has no repercussions. Neither he nor Ny'alotha rise from the depths despite it being foreshadowed for years that Ny'alotha is a sunken city = at the bottom of the sea. Instead he just teleports to some weird dimension (that never gets fully explained) and promptly gets his ass handed to him. For fuck's sake Blizzard, people wanted a cinematic of a horrifying eldritch city arising from the depths, not a dull "well I guess the Black Empire is here now" impromptu from Ion.
Someone on off-topic forums explained why J.J. Abrams' movies suck and BfA suffers from many of the same problems. There's a large cast but most of it never interacts with each other and all of the focus is on the most hateable characters like Baine, Saurfang and Anduin. Things don't get explained properly and events seemingly happen at random, with no build up or consequences. Danuser, Golden and Kosak are not terrible writers but they fucked up BfA's story big time by promising us an exciting epic and delivering a mediocre, uninteresting slog instead.