Worth remembering is that Deep Dungeons is just more than a quick fix of content -- it's actually infrastructure that will remain in the game permanently.
In a recent interview, Yoshi said this content will continue evolving over future patches with new floors and new dungeons in other locations. So my take on that is that Deep Dungeons will get regular additions just like dungeons, 24-man raids, primals and 8-man raids.
Diadem is also infrastructure that could receive similar periodic upgrades throughout the game's lifespan. And had it not been so poorly designed, I think we'd be seeing expansions to it now rather than a complete reworking.
Aquapolis is also good content infrastructure that can easily be expanded and added to.
This is why I'm excited for the game's future. If DD is good and the Diadem gets fixed, already we're going to have an influx of content that's accessible by just about all of the playerbase while presumably offering a worthwhile reward to work toward. And this kind of content infrastructure (that can easily be expanded without needing to build completely new resources) appears to be a trend that SE will focus on moving forward.
Fun + useful is the holy grail. A lot of the progression content in this game is based upon repetition, memorization and a whole lot of dying, which isn't always "fun" (but totally pays off when you make good progress). So I think people are really going to love having less-stressful content that's still mentally engaging and also rewards you with a solid weapon for a primary or backup job.Overall, I think it's more intended to be "fun" than "incredibly useful".
Another thing about DD -- it's content that longtime players can do with people who are virtually new to the game. That's pretty big for recruitment. I have a couple of RL friends who might start playing just because we'd be able to run that content together. Also, the ability to level jobs in DD is going to make the leveling grind much more enjoyable than it is now (for new and old players alike).