For all its lack of originality, the storyline in the Warcraft games is fun - I've been a fan ever since Tides of Darkness back in the day, and (as evidenced by the series' popularity) so have millions of others. So it's rather irksome to be engaged in a discussion about the progression of the storyline via WoW and encounter some smacktard who jumps into the topic, complains about the lack of a "proper" Warcraft IV, and then vanishes into the night like an overly opinionated and misinformed internet Batman.
In case you were curious, the answer to the question, "When will we get Warcraft IV" is "Probably once the RTS team is finished with StarCraft II," but that isn't the point here: When it comes to moving the saga of Azeroth along its path, World of Warcraft is Warcraft IV, even if people don't want to accept it.
With WoW, Blizzard finds itself in a curious situation. It's a game set in a pre-existing universe, not one constructed from scratch for the sake of the game (like Aion or EverQuest). But unlike licensed games like Lord of the Rings Online or Warhammer Online which either retell the story as it already happened, or exist in an alternate non-canon reality where the actions and events that happen in-game don't affect the "main" storyline, WoW is the continuation - and end - of several plot threads from the previous games in the series. And for some reason, people have a hard time accepting this.
On some level, I can sort of understand where they're coming from, and why there's such a knee-jerk reaction against having a popular story continued in a popular MMOG. There's a widespread perception that MMOGs don't have any sort of story whatsoever, and that the games - particularly WoW - are filled with one "go kill twenty wolves" quest after another. That's another popular misconception in itself, but we don't have to address that here; after all, even killing twenty wolves can make sense in context.
Another argument prods at the idea of the genre's persistence (and hundreds of thousands of players). If a small tribe asks you to save them from rampaging war bands of orcs, and you do so and are heralded as their Eternal Savior... can you really internalize that even while knowing that the orcs will respawn ten minutes later, and that every other level 75 character is their Eternal Savior as well? While this argument is made slightly less relevant by WoW's phasing system letting players actually perceive the world changing thanks to their actions, it still comes down to a matter of suspension of disbelief. It's the same reason that Nathan Drake can gun down hundreds of goons while searching for lost treasure and still be a loveable roguish good guy rather than a horrible mass murderer.
But the perception that MMOGs can't tell a storyline, let alone continue one from previous games in the series, is absolutely ludicrous. In early World of Warcraft, we met a disgraced former hero in exile who was living as a hermit, and helped him try to convince his estranged son to leave a dangerous and fanatic order of paladins. Over the course of the quest chain, we learned about his history with his family and the terms and reasons for his exile, we fought alongside a son who had renewed faith in the father he'd once thought a traitor, and we watched as the story ended just shy of redemption. Said former hero has grown from minor (albeit fan-favorite) quest giver to one of the central characters of Wrath of the Lich King.