WoW’s writing, right now, in 2018, is a lot better than it was in 2004, if you look at the actual quests, dialogue, plots and so on.
However, relative to the standard of writing in games in general in 2018, it’s still about where it always was, which is a sort of pulp-y “WWE”-level of writing. It’s got a bit more going on now, but not a huge amount.
Interestingly the low-point of WoW’s writing isn’t 2004, though, it’s 2010, with Cataclysm. WoW’s writing steadily improved from 2004-2008 with vanilla, TBC, Wrath, then some kind of change in the writing team and writing style in 2010, with Cataclysm, just flushed that all down the loo. Cataclysm has absolutely the worst writing WoW has ever seen - cringe-worthy, even borderline racist in some cases (how they managed that in WoW is a separate discussion, and it ain’t about Orcs or Tauren even) and really profoundly stupid and tasteless. The sort of apex of awful is the storyline of the Uldum zone, which was supposed to be this totally awesome zone thousands of years old, with engines of the gods (titans) and so on, and actually? The whole thing is a bad, stupid, unfunny re-telling of Indiana Jones and the Lost Ark, with the player cast as a useless sidekick next to “Harrison Jones” - WoW’s thankfully largely now forgotten Indiana Jones stand-in. It is truly horrible shit. A normal person couldn’t fuck it up this bad, I think it would have to a remarkable combination of large amounts of weed + frat bro culture + just being juvenile dicks to do what they did. And even late Cataclysm was terrible with “Green Jesus”.
Since Cataclysm, WoW started improving again, gradually - MoP was a big step up, WoD was a mess, but not because of the writing, per se, Legion was excellently written for an MMO expansion, with consistent themes and characters, stuff that made sense, cool scenarios, exciting plot developments and so on. BfA, we shall see but hopefully it will improve on Legion over time.
This isn’t actually a writing issue, per se. This is because very late in the development of the expansion, one of the Blizzard high-ups who has since left (I forget which) demanded they change the plotline of the expansion. It seems like it was originally about ACTUAL time-travel (which happens fairly frequently in WoW - what with the Chromatic Dragon flight and so on), but then was swapped to an alternate universe after too many decisions had been made. So a lot of stuff never made sense.
Then the second half of the expansion was rushed out because they wanted all hands on deck for Legion.
Legion was actually very far from TBC - it’s about finally stopping the demons when they try to invade, and then instead invading them, and I think in the long run it will probably be regarded as WoW’s best expansion, or best after Wrath (which is rather worshipped). It’s quite expansive in terms of places and events (shockingly so, even), and there are some very memorable dungeons and raids.
It’s actually even more solo player friendly than WoD, but without locking you away in a base by yourself (which was always an interesting decision in an MMO), with stuff like World Quests replacing Dailies.
I can see why you might think that, but that’s not actually true. EverQuest and FFXI had little soloing and shoe-horned you into raiding. Dark Age of Camelot, however, had an endgame focused on RvR, group content, and crafting, with raiding as just another option, and not one that got you ahead, particularly. But DAoC was still not solo-friendly, just not solo-hateful.
WoW actually got there accidentally - the focus on quests wasn’t in the early alphas/betas for WoW, but the tremendously positive response the devs got to quests and quest leveling made them put more and more and more in as WoW developed. Still, it worked out, and is why WoW had millions of players, not hundreds of thousands.