First of all, I'm not a content locust, completionist, or anything of the sort. This colors my opinions, but perhaps offers a somewhat unique perspective.
For me, and MMO is about the people. I don't form very many relationships with others, but the few I do form I value. I actually can't get very motivated to jump through quest hoops for more powerful weapons or armor. While I don't find the seasonal event quests to be particularly enjoyable, I do like the unique gear rewards, and you can find me sporting my Lightning costume fairly often, and I like to assume the role of a member of the Guardian Corps by assisting low leveled players in taking down FATEs in the newbie zones.
Anyway, One of the difficulties in content is, like another poster mentioned, that you're playing on rails, and those rails are fairly narrow. Even if you're not picky about what you do, arranging a consistent stream of fun for a group of, say, 5 players is pretty difficult. Especially if some of those players are new. Those players won't remain new for long, but in the mean time it leaves a fairly sour taste in your mouth if you have to leave someone behind to *do* something.
A second, and probably deeper, issue is repetition. Most of ARR relies on memorization. Yoshi P mentions the words "memorize" and "pattern" several times with just about each introduction of something new. The game is made of patterns and you have to diligently follow those patterns to succeed. Once the pattern is solved for, the encounters lose their challenge, or at least their mentally stimulating components. You always know exactly what you're going to get. This is also why people can advertise that you must "Know Fight" in order to play. The magic of the battle is gone. They want their Myth, Drops, or whatever, and that's all. I wonder if these players are even having fun at that point.
What I think the game needs, content wise, is more to do that can lead to the unexpected, or at least is not completely telegraphed after the first success. How about a dungeon where you don't know what the boss will be? Or even what the rooms will look like? Imagine a randomly generated cave complex with who knows what lurking inside. You don't know which way you have to go, or where every treasure chest is located. Guard some of these chests with powerful bosses, and allow them to be skipped. Do you choose to fight the boss and get the lewts? Or do you pass by? Maybe you can't defeat this one, but all is not lost, and maybe you learned something about how this boss operates for future reference. The important thing, though, is that you had a choice. You controlled your destiny and direction, even if in a relatively minor way.
Completely separate is a lack if interdependence. My FC is very friendly and very close, comparatively speaking, and we struggle to think of anything we can do for each other anymore. We used to make each other tools and gear all the time as we leveled up, but now we're all 50 and our opportunities are pretty thin. We satisfy our kindness more often than not by helping people who are outside of the FC when they're troubled, but even finding these players is a rare treat.
Getting to level 50 as a crafter is so easy it's almost trivial, and a lot of players who are waiting on queues have nothing better to do with their time than level every craft in existence, even if they don't enjoy that. What's even worse is that leveling some crafts (carpenter, weaver) is almost required to craft at all. Lack of desirable goods combined with a player base that can already do everything not only equals a dead market, but limits even free exchanges between friendly players. I think players need more opportunity to pursue the things they like. DDs (or DPS, if you prefer) especially need an activity that they can engage in that they enjoy that isn't reliant on queues, though everyone could benefit from some more accessible content. Treasure maps are excellent, but a limited resource, and I'd like to see more content like this where exacting party composition is not as much of an issue and people are free to pitch in just because they feel like it.