Legion has the elements to be the best World of Warcraft expansion yet. There's a new hero class representing a class fantasy people have been wanting for years. Classes are getting seriously looked at to make sure that every spec has something cool and unique. Class quests are not just coming back, but becoming a central part of the expansion's gameplay. The story involves the long-foreshadowed confrontation with the Warcraft universe's biggest villains. We're seeing a recommitment to 5-player dungeons, but not at the expense of raids. We're seeing a new form of questing and leveling that will dramatically change the linear nature of everything that has preceded it.
It looks really great.
While I wouldn't say it's the only reason to worry (everyone is going to have their pet issues, and of course some peoples' tastes are diametrically opposed to others',) I think the big concern is how well Blizzard can maintain it.
Warlords of Draenor was a disappointment. There's been an anemic endgame, and while Blizzard has made some valiant efforts to spruce it up, you can't really argue with the fact that it kind of dropped the ball. The concern, other than the dip in quality (though, for the record, we're talking relative to Blizzard's usual efforts, which is a high bar,) is that Warlords was actually hugely popular and successful when it first began.
Questing through Draenor was probably the best we've ever seen. The experience of traveling through Draenor and playing through the story, as well as the clever Timeless Isle-style treasures and events littered through the world, was great. The dungeons were tuned excellently as well - not the breeze of Mists heroics or the brutally punishing difficulty of Cataclysm ones, but a nice robust challenge.
Really, where things fell apart was the lack of new content. The out-in-the-world content was not so hot after hitting the level cap. You basically had the daily assaults, which were kind of blah daily quests without much sense of progression. You had the garrison campaign quests once a week, which were ok, I guess, but didn't really feel as urgent to the overall plot as things like the 5.1 Landfall quests. And also, let's just take a moment to say that the Iron Horde was just not a very compelling set of villains. These were all figures we had already dealt with in one way or another, and even if it would have been cool to interact with these old lore figures (and it would be fair to say that it could be,) we didn't actually do much of that, instead spending more time with new characters like Yrel (and don't get me wrong, I think Yrel was great. But would I make an expansion out of hanging out with her? Probably not.)
The fact that there were only two raid tiers (if we're counting Highmaul and Blackrock Foundry as two, then we also have to count Mogu'shan Vaults as its own, and now it's disappointing that we had only three instead of the typical four) made it feel like we were really only getting two-thirds of an expansion. And even after Blizzard acknowledged that players want more 5-player dungeons, they still didn't add new ones with the later patch.
Part of why this was so frustrating is that Mists - certainly flawed, though I think its reputation is improving as time goes on and we gain perspective on it - sustained things so well (until the end.) Mists gave us several interesting content patches, each with cool new stuff and a developing story. Mists had four major patches, and the only one that felt underwhelming was 5.3 - but there was still some cool stuff to it. Warlords theoretically had two major patches, but 6.1 made 5.3 look like 5.1 (man, if you don't know what patches those correspond to, that might be the most meaningless sentence ever.) Really, Warlords had only one major content update, and while 6.2 was perfectly substantial enough to earn the name "Major Content Patch," it was really the only one in the whole expansion.
So how do you sustain interest?
To be fair to Blizzard, it's not really possible to do it 100% of the time. Even Wrath, now considered by many if not most players to be the best expansion in WoW's run, kind of lulled a few months after 3.3.
But here are some elements that I think can help keep people interested in the game:
Currencies from easier content: Doing Emblem runs in late Wrath kept players running those old dungeons. It became less of a challenge to beat the dungeon, but more to see how quickly you could do it. There were Azjol-Nerub runs that lasted less than ten minutes by the end. It also helped get high-geared players running with low-geared players. Was there ugliness? Sure. But it also sustained a large pool of players to run all the content in the expansion.
Easy lowest raid difficulty: In Wrath, players were easily pugging the first half of Icecrown Citadel. 10-man normal was truly an appropriate difficulty for friends-and-family guilds. These days, perhaps simply due to the designers trying to come up with new mechanics, normal raiding is very complicated and just not feasible if you want to bring everyone along. I've argued that you should be able to do essentially private LFR runs, which could solve this issue.
Gradually un-rolling stories that the player unlocks: One reason the garrison campaign failed to live up to the 5.1 story is that it was entirely based on time. Yes, the 5.1 quests were gated behind certain reputations levels, and that reputation was based on daily quests, thus gating it by time, but it still meant that it was the player's actions that allowed them to experience more of the story. Get people invested in the story by having them feel like a real participant.
Self-Expression: Transmogrification was just one of a few features that were introduced in 4.3, along with the new Darkmoon Faire and LFR. While LFR might edge it out, Transmog is a contender for the biggest thing introduced in Cataclysm. This is an RPG, after all, and the whole RP part of that is based on the idea that players want to be able to create a character. Due to the limitations of making a digital game, there's not a ton you can do to make your character distinct. Transmog gave us a fantastic tool to do so. One of the biggest flaws with garrisons was the resolute domination of function over form, when a lot of people had been hoping to use it as player housing - something that historically has been a means to express oneself.
Keep the world engaging: Warlords of Draenor did kind of leave us stuck in our garrisons while we queued for stuff. The reputation grinds were pretty underwhelming and boring (because they were truly just grinds) and the apexis gear was just not compelling until Tanaan Jungle opened up. Legion could have a good edge here, given that the level scaling will make every part of the Broken Isles appropriate for 110s. My advice: use it!
So that's my thoughts on keeping the game fun and engaging.