I feel I can say this before it's released and time will prove me right. This is definitely not the place to voice such an opinion, fanboism aside. But for as long as I've been playing this game it has gone from bad to worse. Numbers don't lie and the fact of the matter is that WoW has peaked: it peaked during Wrath and has been going downhill ever since. I'm not talking about some subjective view about my personal preference, I'm talking about hard facts. Since WotLK, WoW lost approximately six million subs, equivalent to 50% of their player base. That's as far as we know, to be fair, since Blizzard decided that subscription numbers were irrelevant and stopped reporting them. How convenient.
So let's pretend for a second that evidence matters and draw appropriate conclusions: what happened at that point? What made this game grow the way it did, then fail the way it did?
I would argue that WoW failed when Blizzard decided to bend over backwards to accommodate all manners of complains. At its core, an MMO should be about progression and character customization, about overcoming challenges and having to learn the intricacies of your class in order to do so. But this is an MMO that does not cater to the MMO-audience, it attempts to cater to all. It has become easier and easier as time goes by, and as a consequence subscription numbers continued to drop. To deal with this, instead of upping the difficulty, polishing the content (old and new) and making sure classes are intricate and fun to play (and pvp is balanced, god fobid), WoW's development team has resorted to all manner of gimmicks: archeology, dailies, garrisons, pet battles... something new and shinny which could be marketed... DKs, DHs, Monks... and which would hopefully entice people to play... new abilities which are useless but new! and will lead to gutting... and WoW continued to deteriorate.
I've seen nothing to demonstrate otherwise. I recently resubbed prior to legion then unsubbed one month later. There is absolutely nothing in WoW at the moment that indicates, as far as I'm concerned, that the development team understands their own game and the appeal it can have if done properly. Long story short: Legion will fail, as has every expansion since WotLK (arguably TBC, as it saw greater growth than the former), because Blizzard does not understand what the MMO-target audience, or MMO player, is all about.