Sorry for a late reply — caught a random ban on the forum previously. 
Yeah, I know, seen it.
True. On the other hand, looked like they tried to reanimate a dead horse, namely the Old Horde, since the new one wasn’t what it used to be anymore.
That’s one of the big complaints about SLs in particular that I’ve heard from players.
That’s true from a lore perspective, however, there’s more to it than an anticlimactic boss (speaking of which, even Rafaam with all his cliches would have done the job better, really).
Let’s take a look at Sylvanas, who’s been out there since WC3. She’s been a twisted and… complicated character (remember her plans for her ‘Little Moon’ Vereesa and so on?), yet an interesting and well-developed one, who has undergone some evolution and all that. What have they done (apart from turning her into a ‘yas, kween’, yet another Tamsin 'Mary Sue' Roame and such, if you know what I mean)?
First, by retconning it so lazily, they have eventually thrown away any character development since the ‘Edge of Night’, if I remember the name correctly, completely devaluing it all and Sylvanas herself, only to be replaced by what? Some shoddy, hastily cobbled together excuse of a story?
Speaking of which, second: if it was all just such a plausible ruse by her that we all believed it (maybe because it was considered genuine before the retcon? I mean, they probably wrote all those books, stories and all for a reason), how would such a masterful schemer, manipulator and pretender fall for Zovaal the Big-Nippled Bozo’s trick, when even someone with the wits of Guff wouldn’t? Either he must have done a better job at manipulating her, actually living up to his portfolio (oh, and what potential for it there was…), as opposed to what he demonstrated, or she shouldn’t have caught such an ‘idiot ball’ and realised his rather obvious motivations and ways much earlier — elsewise no amount of the so-called ‘suspension of disbelief’ allows to accept this lousy kind of storytelling.
PS Come to think of it, the usurpation of the previuos lore by another ‘lazy “It was me the whole time!” snore fest of a big boss’ — the one with dreadlords, you know — doesn’t look much better, including the quality of this work, but that’s a separate question, I guess.