Looking at complaints on "undead high elf not fitting Horde's theme" I tend to disagree.
Thrall's Horde's theme? Yes. But that era had a different Sylvanas, too, personality-wise.
But since Cataclysm we've been heavily drifting towards the Old Horde faction fantasy, not just with Garrosh (though he did play a large part), but more with orcs now being warmongering and bloodthirsty regardless of demonic blood.
And what was fundamental with the Old Horde lore? Warlocks, necromancers and undead.
Some of the most powerful and influential Old Horde figures were physically frail, un-orc'y, scheming evil sorcerer types. Namely Gul'dan, Teron Gorefiend and even BtDP era Ner'zhul.
Sylvanas fits the team perfectly if you ask me.
Undead high elf? Teron was an undead human (possessed by an orc's soul, but still). Oh and he wanted a lil' personal world for himself and his death knights, not unlike Sylvanas' kingdom of undeath.
Burning Teldrassil out of sheer spite? That's exactly what Gul'dan did in Legion audio drama, torching a ship full of innocents just to piss off Khadgar. And I won't even mention mass sacrifices and such, you don't earn the Destroyer of Dreams achievement and title by picking flowers and petting squirrels.
Raising her own troops as undead? Any necrolyte's and death knight's tuesday.
Treating whole of her faction as disposable cannon fodder? Ner'zhul "I dumped half my Horde to die on Draenor" says hello.
Even her looks are pretty close to Warcraft II's death knights. Skeletal armor, cloak and hood, an undead steed etc. Only using bow instead of a staff or a melee weapon.
So in my opinion, Sylvanas' Horde is pretty much true to Old Horde faction fantasy. We had a Grom/Orgrim/Blackhand expy combo (Garrosh), we had a rather poor substitute for Zul'jin (Vol'jin), now we have essentially a female Teron Gorefiend on the throne, complete with vibes from Gul'dan's petty cruelty and Ner'zhul's selfish determination.
Given that BtDP Horde campaign was aptly named The Burning of Azeroth (and Kul Tiras was sacked along the way), there's not much room for the coincidences, and personally I have no problem with it being that way.