After having played the early game from a new player's perspective and the late-game a few months ago, I think I can point to exactly three main reasons as to why WoW declines:
During the leveling game you level too fast. And I am not talking about a little bit too fast. If you don't have heirlooms and don't use rested EXP and don't gather resources and don't do dungeons and don't do battlegrounds and never kill more mobs than necessary for quests (e.g. explore), you level a bit faster than would be good!
But people like to actually do a dungeon every now and then. Some like to do battlegrounds. Many have gatherer professions, many kill more mobs than necessary. Many will use rested EXP and so on. In practise, it is hard to not outlevel the content! And even if you do fight mobs at the correct level, they are often too weak to allow you to use more than two or three of your abilities. You just don't have a chance to play your character! That's frustrating.
Now, combine this with very linear stories that are often even interesting enough that you want to do them! These quest lines often are, in fact, good. And this means that you don't want to skip them. Especially since skipping means going to a new zone, as the entire storylines are linear. But if you don't skip them you mostly end up one-shooting grey mobs. That's not sustainable for 84 levels!
The jump in the game's philosophy, story and quality from 60 to level 61 drives players crazy.
The relentless focus on raiding is bad enough for players who don't want to raid during endgame. But the difficulty jump is not from this world. At 84 you were rushing dungeons with heirloomed tanks and healers. It was 100% irrelevant what buttons you pressed or where you moved.
And then suddenly at 85, you enter a really hard raiding game designed for long time raiders with lots of experience. To be fair, Blizzard is doing something about that right now. I don't know whether this will be enough, though. The tiered system that gives you second-highest level rewards for farming LFD dungeons disincentivises doing older raids.
There are many, many smaller problems, too. Some of them not even that small. But these three problems are, in my opinion, the main reasons for WoW's decline.
It is especially disastrous as players in the past made a new twink when the endgame didn't interest them. But the leveling game is maybe even more problematic than the endgame right now. If you don't want to raid for some reason and don't enjoy killing grey mobs, then there's actually not much choice, but to unsubscribe and get Rift.