Deathwing. He went from a world breaking mastermind to an absolute herp derp in 4.3, throwing himself into losing situations over and over again. Blizzard realised that they have made him so powerful in the beginning that they had to turn him into an absolute idiot in the final patch, otherwise we couldn't have killed him at all.
Garrosh and Kael have been wasted by being turned into villains in the first place. Garrosh could've been a great leader of the Horde for a much longer period of time, now all we do since his death is passing the bone of the title warchief to another character nearly every new expansion. Kael was a great character in WC3 who had his own campaign, showed him as a compassionate character who really cared for the good of his people, now ended up serving the ones who made the Scourge in the first place. And apparently for some reason doesn't give 2 shits about the people of Silvermoon anymore. Despite being so proud of Quel'Thalas as part of his dying quote... his character is such a mess.
Deathwing, easily. Like comically not even close.
All of this lore and build up for DW and he barely made an appearance during his whole expansion and then the raid was barbie pony adventure.
Kael'thas because everything was just a set back.
For me it has to be Garrosh and Deathwing. But I'd vote Garrosh since he was done the least justice.
He is exactly the kind of character the Horde needs and which Sylvanas tried to replicate but failed. The factions need shades of grey for the faction war to be believable. Pure good and evil doesn't work, it's not interesting.
Garrosh' Horde was the best. It would be nice to him bring back somehow. Give him a redemption without neutering his personality.
"We're more of the love, blood, and rhetoric school. Well, we can do you blood and love without the rhetoric, and we can do you blood and rhetoric without the love, and we can do you all three concurrent or consecutive. But we can't give you love and rhetoric without the blood. Blood is compulsory. They're all blood, you see." ― Tom Stoppard, Rosencrantz and Guildenstern are Dead
Archimonde, azshara for the moment, deathwing, and some others abruptely killed/disapeared when they should have a return later, like kel thuzad. He did it for anubarak, why not kelthuzad ??
We didn't even need him(or Gul'dan on that matter) to have Legion.
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Deathwing at least got his own expansion. Poor Kil'jaeden lived and died in shadow of his servant, Gul'dan.
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Kel'thuzad is in his phylactery and he can come back whenever Blizzard needs him.
Deathwing had an awesome setup and interesting backstory, but was handled with probably the worst execution of any character in anything I've ever witnessed.
His dialogue, voice, story, impact, raid, you name it, were all dreadful.
Like you almost had to try to intentionally make it that bad with that good of a setup.
Illidan was going to get my vote before Legion but now I'm going with Kael'thas. He would've been an interesting blood elf leader. But it is most certainly too late now.
If alternate realities count, everyone in the Iron Horde except Grommash, who at least got a little something in the mag'har scenario. ESPECIALLY Orgrim Doomhammer.
Pretty much all of them are a waste, for the sake of raiders getting their 1000 boss to kill.
In fact at vanilla we delivered his phylactery to a man of the argent crusade who seemed not very sane after delivering the good, and supposedly disappeared after.
(yeah, i remember this one, I did it start BC when we wanted to do a rerun to finish the few boss when werent able to finish at vanilla) :P
Loken....for all the lore implications he had
Dungeon boss
I'd say Vashj. Kael'thas was wasted and turned into a loot boss but he still had a big role in the story, and did a lot of stuff during the expansion!
Vashj did literally nothing, and ANY Naga could have taken her place.
All the others were perhaps misused or underused, but Vashj was literally a raid boss that was there, and died, that was it.
The great hero of TFT who did so many awesome things just...got killed by adventurers without doing anything the entire expansion.
Deathwig. Cunning manipulator and mastermind ultimately amounts to just a big scary dragon in Cata.
It's unfortunate that Kael went evil but I can't really think of much more they could have done with him AS a vaillain. And ILlidan got brought back in Legion.
Kael and Azshara easily, Kael just got well destroyed as a character in every possible way. And Azshara could have easily carried an expansion herself instead of just being another old god tie in. But massive push towards Deathwing as well, he was easily the most fun part of Cata with the random zone burnings and appearances, he felt menacing and then the ending was so boring. Granted I can't think of how to deal with a giant magma dragon but it was deeply unsatisfying how they did.