World of Warcraft has a serious problem since its beggining, which has increased over time. The main problem is that it's a limited format when it comes to storytelling. In Warcraft III we play and see the Heroes all the time. In WoW it is the opposite, we are nobody.
WoW wasted a lot of characters just to give us enemies to kill... for example, everyone remembers The Burning Crusade (12 years ago) as a good expansion but in terms of story it's a disaster. All the expansion story is told subtle between missions but it's not in a clear way. They completely wasted Kael'thas (more or less because it's not illogical what they did with him but it's a wasted character anyway); Lady Vash was treated even worse... We go to kill her and that's it.
Three of the main characters of TFT never met again (Illidan, Kael'thas and Lady Vash), never talked to each other... How many times do we see Illidan before assaulting The Black Temple? two? three? and only one of them is physically Illidan, the rest are "illusions".
As you can imagine this not only happens with main characters... Minor characters were also completely ignored:
- Nazgrel, who was supposed to be the Chief of Orgrimmar Forces and what does he do in WoW? He was in Thrallmar doing nothing.
- Rokhan, did nothing important and had no dialogues until Battle For Azeroth.
The story doesn't develop as it should, disappoints to see unsatisfactory conclusions. Why do you think they brought Illidan back in Legion? Because nobody liked what they did with to him in TBC, therefore they had to rewrite his entire story. If you remember, in Classic, Malfurion said at an event that Illidan was crazy because he remembered his defeat against Arthas again and again. All of this was changed in the novel and in Legion to make Illidan the good guy and that we were the bad ones.
Not to mention Wrath of the Lich King... Muradin said in Icecrown that he has a pending issue with Arthas and he doesn't appear until we finish Arthas... Where have you been damn dwarf?
That's the MMO problem, you have a legendary character like Muradin throwing you an epic speech and then he does nothing... it's disappointing.