They have no depth, and no charisma to back up their lack of depth. Also, specifically from a horde perspective, I will never be able to get past the entire questline where hellscream is moping around being a little defeatist bitch becuase he's not as great as his father and their people are doomed. You do an entire questline helping them out and at the end of it, he gets even more despondant and whiney becuase it was you that did all the work and not him and oh what a worthless creature he is. Then with no warning wrath comes out and he's all gung-ho rip-roaring kill all alliance with no transition story in between. And Varian wrynn doesn't even have that amount of story (in game) going for him, he just shows up out of nowhere trying to start fights.
It's very true. Garrosh and Varian emerge in Wrath almost fully-formed. There is a questline with Garrosh in TBC, but he doesn't play a major part, and Varian just shows up (and takes credit for Onyxia, to boot).
This is in stark contrast to one of the other major characters in Wrath: Tirion Fordring. Unlike Garrosh and Varian, Fordring is introduced in 1.0 in Eastern Plagueline. He has one of the best and most-loved questlines in the game. It's also a very personal questline for the character, and shows the reasons he decides to reforge the Silver Hand. In a lot of ways, Wrath and the Argent Crusade are sequel to those quests.
I think those quests have a lot to do with how popular Tirion Fordring is. We got to see the character at the beginning of the story, to sympathize and empathize with him. By doing the quests, in a way we were responsible for the character, and the way events turned out. We are more invested in the Argent Crusade and Tirion's storyline.
Tirion illustrates how important doing is to a video game. Garrosh and Varian illustrate how weak merely telling is. The strength of games is that we are not passive consumers, we have to do something to advance the story. That has the potential to make us more involved with the story, and make it more important.
In the abstract, I understand Garrosh's and Varian's story. But I am far more interested in Tirion's story, and that is almost entirely due to the original questline in the Plaguelands.
Would Varian have been more successful if there had been a questline to restore him to the throne? I think so. Similarly, a questline showing Garrosh's conversion to a more aggressive stance would have helped a lot.
There are two major lessons here. First, in an MMO, doing is always better than telling. Second, major storyline characters really benefit from being introduced early and fixing the player's sympathy, before they actually undertake a major role.
There are a lot of other examples. For example, consider the player base's attitude towards High Overlord Saurfang, Chromie, or Bolvar Fordragon vs that towards someone like Rhonin. Early exposure in a more trivial setting really benefits the character when it comes time for them to take the spotlight.