I compared Arthas because he was done more injustice than Vol'jin. Arthas was the most iconic figure of Warcraft to me and he got defeated by a single paladin's prayer atop ICC. It doesn't get worse than this.
Varian was much more of a warrior type for the Alliance than Vol'jin was for the Horde. Vol'jin was always a "filler" figure and never really felt like a leader, whereas Varian was a true leader to the Alliance. Their importance within their respective factions is impossible to compare. Imagine if Mekatorque was made the leader of the Alliance while Thrall was chief; would you expect him to have the same type of heroic death you'd expect Thrall to recieve?
Your expectations were too big. Mine never were and that is why I have no problem with what they've done with Vol'jin.
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The entire Horde isn't related enough to eachother. The entire Horde is a band of factions with different attitudes, different aspirations, different morals and intentions. That doesn't make the plot any less credible.
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That is how you feel, in addition to certain Horde fans and troll fans. Just because you think he didn't get the death he deserves doesn't mean everyone feels the same way. It is literally the same people repeating how they're not satisfied with it all over the place, making it look like there is more people that don't like it than there actually is.
Vol'jin fits the role of advisor perfectly, but not the role of a remarkable leader that excels at combat. Those that expected the Varian-type heroic death for Vol'jin are delusional; Varian went out with a bang because he always came in with a bang. Vol'jin on the other hand, as a diplomat and spiritual-type of leading figure, had more chance to die of infected toenails than a fight because his character was never focused on direct combat(similar storytelling can be observed by the AU Velen who literally sacrificed himself to purify a Naaru by walking into a beam of void energy, instead of going in with a staff and battering Ner'zhul to death with it).