Rng, AP and the grinding needed are the biggest problems with this expansion. Most of the people I know who still like the expansion are casuals who don't take things too seriously or the folks who are still addicted and play 15+ hours a week. Even my friendships that have kept me in the game besides raiding are getting strained because I often don't farm enough mythics and other things to maximize my gear for raiding. I have one 110 character with 3 specs at 35+ AP (as I swap roles often enough). Every minute you spend on an alt or not your main spec, is a minute not spent maintaining the grind for your main. It made little sense to level up your off-specs until you got your AK buff up to 25 but that locks you into one spec.
It became obvious to me when they detailed the artifact changes for 7.2 a couple of months ago that it made no sense to farm AP unless you were just that addicted, mythic raiding (and need the buffs to kill bosses) or your class/spec was so imbalanced that a 10% dps buff would help. Only to have it nerfed tomorrow with 7.2. Oh, you get your points back but that just means you'll be about a month ahead of everyone else with a nerfed buff.
In the end, the grind of Legion just never ever ends. Add in the nerfs forcing respecs/re-rolls with another AP grind, master loot requirement changes screwing up raids (all to supposedly benefit pugs), people begging for loot from people in lfr since you can trade gear now, and legendary item RNG so bad such that mythic raiders are re-rolling classes if they don't get best in slot items for their spec instead of farming more 5mans/bosses/world quests to get a new legendary item.. It is a recipe for burn out. At this point, I figure this is my last expansion and I doubt I'll make it past the summer.
Here's my take on the infinite grind... I think they've been trying for years to implement something that EQ/EQ2 has had for about a decade, and that's Alternate Advancement, a way to improve your character beyond just the levels. They fiddled with gearing upgrades (normal vs heroic vs mythic vs warforged of each, titanforged of each), fiddled with different ways of enhancing gear (gemming, enchanting, reforging), etc, fiddled with upgrade-able gear (think legendary cloaks/rings, old kara/ICC rings, etc) but kept focusing a lot on gear itself which IMO was a silly mistake. This expansion they've gone slightly saner and made you enhance a weapon that's more integral to your character, which is sorta like upgrading the character, by way of a point system that technically doesn't have a limit (beyond how many points they give in the tree itself), which is much closer to EQ1's AA system of near-infinite grind for infinite advancement system.
That's all well and good but they've run into a problem with WoW in that probably 90% of people have more than one max level character, and in addition they have multiple specs for each character that now has an infinite-advancement tree. And the worst part is this will all go away next expansion so it will be even more meaningless than gearing up to begin with (something that was a very dramatic shock to most people that hit hellfire back in BC and invalidated all their raid gear). Blizz should have incorporated an AA system a long, long time ago, giving you a way to make a single 'main' that you could no-kidding focus on, instead of logging in once a week for your token raid (at whatever level you raid at) and then go back to gearing up xyz alts you happen to have. Starting this up this late in the game (pun inteaded) is going to give people a sour taste in their mouth if they've grown to enjoy a half dozen characters that now they have to abandon to realistically play one at a level that Blizz has now blessed off on.
EDIT: Forgot to add, Blizz actually had an alternate advancement system back in Beta (like vanilla's beta), they abandoned it somewhere around the lvl 30-35 horde push, and IMO it was a huge, huge mistake. It was really cool, you got points periodically while playing... it wasn't something exposed to the user (though at that time APIs were really leaky so someone coulda made an add-on for it I'm sure). You'd just be fighting and mid-combat a point would be gained, and you could throw it into things like stats, resistances (relevant at that time), chance to hit with x weapon type, modifications to damage types like fire, frost, etc. Neat idea, got dropped, no idea why.