Well in MMO'S Content is King!
Story is content. SWToR's issue wasn't the fact that their gameplay was poor. IT was the fact that their gameplay did. not. exist. There was no endgame. Same problem with Guild Wars 2 which is now funneled into anything that closely resembles endgame as opposed to the widely distributed game they had hoped to create.
FFXIV has no concerns in this regard, there is a very clear endgame in Crystal Tower and Laberynth of Bahamut, at least in terms of projections. Beyond that, there's Primals that continued.
You cite the story of World of Warcraft as a minus and their gameplay as a plus. But in their introduction the opposite was true. Their creation and expansion onto the Warcraft Universe drew in it's early adapter audience, and their endgame was almost as sparse and lacking as FFXI's was on launch.
To clarify, I'm not saying the content can ignore being fun. But I do want to correct the conception that story was mostly ignored in World of Warcraft, which its success is attributed to a grand ton of factors, most of which are merely speculated upon as a continent argumentative scapegoat and rarely are they accurate.
If anything, many cite the fact that Yoshida talking to many of the Blizzard developers of WoW was a negative.
So this is all a mixed soup of deep-seeded opinions that, honestly, I hope Yoshida pretty much makes his own judgement on. But as far as I remember in his older interviews, his prorities are fairly clear:
This is a Final Fantasy game.
This is a Role Playing game.
This is a Massive Multiplayer game.
In that order.
Final Fantasy's strongest suit is Storytelling. If they let that suffer, they lose their install base. So they absolutely, cannot neglect that. When they called it Final Fantasy, that became a default expectation for many fans, MMO or otherwise. Now, I understand your emphasis on game-play but I personally think it's foolish to go for WoW's numbers or WoW's exact approach. Too many have tried and failed miserably duplicating that route. Instead I feel Final Fantasy XIV: A Realm Reborn remember its namesake heritage and expounds upon it to work in an MMO environment. If they create game-play deeply seeded in the lore and environmental feel of Final Fantasy, then the game-play will be a success as well. However it still needs to at least be accessible to the public as well for it to function as an MMO.
It's a tall bill to fill, but honestly I feel confident in their ability to meet it adequately.
Story and gameplay, they're both critically important. However Story sticks to the FFXIV's identity more than WoW's identity.