Disclaimer: this is my opinion, I am stating nothing as fact.
The story in the game is the major reason I play this game. I stopped playing early on in Cataclysm, a big reason for that is because I personally feel the Alliance side of the story was not told very well, having to continue the Worgen story by playing Undead seems silly. Regardless, on to the actual point of this thread.
Blizzard seems to be unable to make up its mind on how to tell a story.
Lets look at this expansion's story telling post launch.
5.1- the Fleet quests and dailies
I never thought I could be more engaged, excited, and impressed with the story telling from Blizz with these Divine Bell quests. They didn't have the best pacing, maybe have a string of a few quests each week instead of tied with rep. The quests themselves were quite awesome though.
5.2- the Thunder Isle
Lots of dailies that took you all over the Island and through the story, maybe a bit too many quests that were too spread out at the end, but they were fun and engaging. The solo scenarios were and are probably the best way Blizzard has come up with the tell the story in a very engaging way.
Throne of Thunder was fantastic, one of the best. It didn't seem very "busy" or "alive" though, just plenty of mobs and bosses standing around waiting to be killed.
5.3- Battlefield: Barrens
Blizzard's first try at a "sandbox" type area.
It failed quite hard in my opinion. The actual story and conflict starts to heavily ramp up, yet Blizzard's story telling ramps down significantly. The first few actual quests, for the Alliance at least were very weak. Probably the least engaged I've been questing, nothing much fun about driving a robot around collecting maps.
The Battlefield: Barrens part was just a grind, absolutely nothing more, just pick up a single weekly quest and then farm mobs until done. The escorts and Commander bosses weren't a bad idea, but could have been better.
There could have been so much more Blizzard could have done with this! Daily sabotage quests, espionage quests into Orgrimmar itself, plenty of stuff Blizz could have done.
5.4- Seige of Orgrimmar and the Timeless Isle
The Seige of Orgrimmar will most likely be my favorite raid instance so far.
The story and cutscenes are great, I love them. The first cutscene in the Vale of Eternal Sorrows is too long and boring though, speed it up. The rest of the cutscenes are great, people might complain there are too many of them, but with a raid like Orgrimmar, so heavy in story and background it needs and welcomes them. The raid actually feels alive and it feels like we are actually seiging a fortified stronghold. I can't praise Blizzard enough about this raid.
The Timeless Isle. Another "sandbox". What happened here Blizzard? Where is the story? Why are we here? What is the point of the Isle? This place seems so detached from everything. There is a full blown war going on and here we are, camping rare mobs, grinding a tedious rep, and... nothing else, thats it. In the past we had a raid come out, and then a quest hub revolving around that raid. Why couldn't we have quests in Ashenvale and/or Azshara? attack Orgrimmar from all sides, deprive them of resources even more. I know we kinda did that in the Barrens, stealing Kor'Kron supplies, but doing something in all the zones surrounding Orgrimmar would have made great quests I think.
Now, all we get is to farm 50 stones a week to see a 15second clip with no sound. Our characters aren't engaged in that at all, just spectators. We don't even get anything exciting with Wrathion past the Legendary cloak.
Blizzard, I miss your great story telling past the launch of an expansion.
The Isle of Quel'danas, The Argent Tournament, The Firelands, Landfall, The Thunder Isle.
All of these were fantastic daily hubs with a great story. Why stop?
I think I can safely say that people like story, they like having some type of structure, they can't appreciate "sandbox" type zones if it detracts from storytelling. a "sandbox" zone is typically go where you want, do what you want. It doesn't work if there is actually very little to do. You can fight the Celestials, once a week, you can do the pet tournament, once a week, after that you're stuck with farming Yaungol for the remainder of the week.
The Isle itself isn't even big enough to truly be a "sandbox" anyway.
"Sandbox" type zones and content CAN work, but not at the expense of story.