Not even a few weeks into Legion's release, normals were no longer relevant content. At this point, if not at the same time, Heroics were no longer relevant content. As it is, for players who started on release day and pushed, regular Mythics are no longer relevant either. Which leads to players complaining they have nothing to do, and developers complaining that players blow through their content too quickly.
I remember a quote from an MMO developer years ago. It went something like this: "Players are like locusts, devouring content and flittering to the next thing." It was a designer complaining about players who sprint through their games. I was, to some extent, sympathetic to their complaint. But years later, more MMOs later, I think that players are only part of the problem. The core design of most MMOs are the problem. Because, when a player says, "I have nothing to do," they don't mean, "I have accomplished every feat and achievement in this game in every possible way." They mean: "There is nothing relevant for me to do."
There are very, very few players who are going to do Loremaster four -- if not more -- times to complete their transmog bank. As it is, few players are going to do Loremaster because it is not relevant or interesting to them. Why? Because the designers have created a game world that makes old content valueless and a waste of your limited time. Worse yet, they communicate to the players through their design that this other stuff is boring and uninteresting, and worse, we don't care about it either.
Most people who talk about this already say that to get players to spend more time on content, it needs to be fulfilling and challenging. But, there's two more parts to it. It needs to be something that the developers appeared to care about and it needs to not be designed and viewed as just a stepping stone to the next tier of content. About 70% -- I'll even wager up to 85% -- of the quests in WoW, even in the new zones, are not meaningful in any way. For the most part, they are even pointless from a narrative point of view.
FF14 does this a lot. As you're getting ready to attack the final dungeon, your player stops to feed some troops. Rally some troops. Tell someone that if the radio goes down, use the signal fires, and a bunch of other things that break the game's momentum. Too many quests break the flow. The intro to WoD is great -- the flow works perfectly and each quest feeds into the next without breaking the mood or pacing. The problem is, the vast majority of WoW does not feel like that. You have breadcrumbs to hubs with the occasional reminder about the over arching quest line. Some of the zones in Legion keep pacing better than others, but Stormheim and Valisharah are probably the worst at it. What does this communicate to your player when you are creating quests that get in the way of your own story/narrative?
The designers created these quests to keep you from the end game longer. Players then treat these quests as exactly what they are: speed bumps. In WoW, the problem is seen clearly with players viewing 1-100 as a giant speed bump, and heirlooms and dungeon queues demonstrate this clearly. But, if you want to see it in a pure, crystal like form, look at what happened when you could level 1-100 in a day or so using invasions. People -- even new players who had never seen WoW's leveling experience -- leaped at the chance to skip 100 levels because they realized even the designers think leveling is pointless, waste of time content, even for new players. Players received that communication loud and clear, and cheesed the hell out of invasions because, with the way the game was designed, that appeared to be the intent.
Players see that designers seem to care very little for dungeon and world content. Instead, they seem to focus on raids. After all, that's why LFR became a thing. So, players follow the designer's lead. The designers have given very little indication that they care about Azsuna or Stormheim, as they are designed in such a way for you to sprint through them and do your best to not get bogged down by time wasting side quests. Each zone is bogged down with pacing issues, and players interpret this in the way you'd interpret a filler episode in a TV show: This isn't important, but we need 24 episodes, so how about Sam buys a Tan and Wash? The audience doesn't care about the episode, it gets added on to the "skippable" episodes list on fan sites, and they move on. Players have filed level 1-100, normals, heroics and most of the leveling experience in Legion into, essentially, where monster of the week episodes fall. Interesting, but a distraction from the overall arc/point of the game.
Which brings us back to the problem with releasing so much tiered content early. If you release three tiers -- T1, T2 and T3 -- players are going to do their best to ignore T1, do the bare minimum in T2 and try and squeak into T3 as quickly as possible. The Normal-->Heroic-->Mythic dungeon grind is economical for developers, but it feeds in to player perception that Normal/Heroic are wastes of your time. If you release all your episodes on Netflix at once, people binge on it. If you release all your content at once, players will binge on it. If you release interesting content that is rewarding to do, then you end up syndicated. And eventually on Netflix.
The over lapping release of Normals, Heroics and Mythics ruined the pacing of the early expansion. And, much like mob scaling, does a lot to cripple a new 110's enjoyment of the content because they can't stop and finish a zone or run the dungeons to get a feel for them. They are now two to two and a half tiers of content behind. They must begin the sprint to be relevant or be left languishing. Once raids release next week, someone who dings 110 will be about four tiers of content behind. The game has now told them: those side quests and world building are wastes of your time. Get in a dungeon. Get your gear. Move quickly or you'll be even FURTHER behind.
That feeling and design is one of my biggest gripes about Blizzard, and MMO, design in general. People play games the way you design them to be played; if you create a game that is a sprint on a gear treadmill, then players are going to treat it like that and burn through the content. Add in the various content gates [order hall campaigns, 7 and 3 day research/work orders, reputation grinding], and you have a game that wants you to have lots to do, but punishes you for doing anything that isn't "gear up as fast as possible."
That's why Star Wars: The Old Republic, Secret World and GW1/2 did well in keeping people interested [at least, until their various problems at end game showed up.] The early/mid games were not just time sucks punishing you for not being an early adopter. You had interesting, challenging things to do in a engaging worlds. MMO design needs to adopt the philosophy that gave us the interesting early/mid games of these MMOs, with the deeper, more fulfilling end games of FF14/WoW. I'm not sure how they do it, but I think that would fix the problem with players always feeling rushed/like they have nothing to do.