Thats not even remotely bad writing. If they made every character like the way you say is good writing, we would have every character being basically the exact same. Right now, we have different leaders with different goals and different beliefs, which is really good.
Jaina believes the horde is shit and is destructive wants revenge for the hordes actions against theramore and at the broken shore. Whether Anduin believes she shouldnt be hellbent on revenge, she believes she should, which is good writing because that makes her a different character, which is good.
Genn believes Sylvanas is shit and wants revenge for events that unfolded a long time ago and after he saw her ditch the horde. Whether thats what happened or not doesnt matter, because Genn believes it happened and thats exactly what makes him his own character.
Velen wants revenge for his son and his people. Hes tired of sitting around and just being the prophet, so now he wants in on the action and wants to take his home back for his people.
Malfurion wants to rescue Cenarius and cleanse the emerald nightmare along with Tyrande.
Anduin wants peace among the factions but also wants the factions united against the Legion.
The dwarf leaders are focused on Magnis return.
See, thats not bad writing at all. If they did what you call good writing, every character would get wiser and stronger due to hardships and every character would be the same. The way we have now is just fine, every character has dealt with different ordeals, and every character deals with them in their own way. They do there own thing and believe in their own ideals. Thats exactly how it should be and thats how it is.