Now that World of Warcraft: Legion is in its fallow phase at the end of the expansion, it's time for a review of the game as it is prior to the launch of Battle For Azeroth.
The idea of acquiring the most powerful weapons that one's Class has, and wielding them against the Burning Legion, was a good hook. The system around this feature, Artifact Power, turned a retention mechanic into a burnout mechanism. The top end raiding guilds, PVP players, etc. saw that maxing out the Traits in your weapon was a huge power boost, so that became required to be viable in top-end content. The grind for AP wore out so many players that they quit playing entirely, ruining many guilds before the mid-level patch hit and forcing many more out of the top-end tier if they wanted to avoid that fate. In short, it backfired- and it wouldn't be the first mechanic to attain the opposite of its stated aim.
The system not only penalized playing more than one character, but playing more than one specialization on that one character, furthering the error of thinking of specializations as separate classes. This version of the game's design had too many class-defining capabilities locked behind the Artifact, some being so important that you could not function properly without it. The game already has too much of a difference between how the game is below the cap and at the cap; this just made it much worse. (Incoming patch hopes to fix this somewhat, but we'll see.)
While Artifacts were only specific to this expansion, it's not hard to see that this was a test-bed for replacing gear as the big carrot to chase. The overall failure of the design means that no ass-pull will be made to keep them up going forward, so they will be put to be and the benefits thereof redesigned into the classes and specializations to be gotten by other means- much as the Draenor Perks were previously.
A failure. There are two factors to the failure. The first is that you could only get them randomly, and only by pursuing activity deemed "eligible" (which included taking the Blingtron 6000 quest, for some stupid reason), again as a mechanism of player retention. (Yes, that damned Straight Out of Vegas applied psychology thing again.) So you could go the entire expansion without getting a Legendary worth a damn, depending on class and specialization. The second is that every class has a set of Best In Slot items that included Legendaries; without those Legendaries, you literally could not compete as a viable teammate to someone that did. Top-end guilds, going into this expansion, rolled multiple characters of the same class and spec; as soon as one of them got the required Legendaries that one became the main and the rest got benched or deleted. That's how bad this was.
So you had no control over when you got them, or what you got, so you had no choice but to maximize your activity so you could pull the lever enough times to get enough Legendaries until you got the ones you needed to actually play your character to best effect. The devs acknowledged this, letting us know that "Bad Luck Protection" existed (each failure to proc a Legendary added to the % chance of a proc down the line until you got one, then it reset) with the first two having much higher % chance rates. As with the grind for AP above, this greatly contributed to player dissatisfaction, burnout, and therefore subscription cancellations.
Which leads to the next thing that wrecked things.
This began in Mists of Pandaria, but it got out of control with Warlords of Draenor and lost all semblance of sanity with this expansion. The idea is simple: instead of being the standard Item Level when a piece of gear drops, it upgrades one or more steps (subject to a cap; as of this post, it's 985). The first step remains "Warforged" and after that it's "Titanforged". So it was not enough to go to the most difficult content and conquer it to get the best gear; you had to farm ALL iterations of that content to maximize your odds of getting the best gear for your character. More grind, more repetition intended as a player retention mechanics, translating to more burnout.
This is not accidental. This is intended.
The reason for so many retention mechanics welded into the core structures of the game stems from incorporation of people from the Diablo team, and a larger shift of the game from being a RPG to being an action game- that's what the former D3 devs brought to the team. You were always expected to have something to do, and something to strive for, solely to keep you subscribed. The problem is that most players want to be done with the game at some point, where they either stop playing for a while or switch to an alt character for a chance of pace. (Or even play another specialization.) These systems directly fucked you over if you did that, until later patches lessened that burden somewhat.
Contradicting this intention was the myriad of class-specific and specialization-specific things to do, experience, and acquire that encouraged playing alt characters. The devs goofed a lot this time around, but it wasn't all bad. That's for next time.