His duty was to lead his people, to ensure their survival and prosperity. In many regards he was successful, others, total failure.
The journey to Outland was a solid idea. It was a source of magic, something which his people needed. It was also a way to start over again in a new land, perhaps a safer land. After all, there are no hostile trolls in Outland. Well, until the Horde showed up there weren't. :)
Still, going to Outland may have been an abandonment of his homeland. He may have worked for the prosperity of the Blood Elves as a race, but is it good to ignore location? He would have dramatically changed their culture by tearing them from their homes.
But this is a difference of opinion and I cannot say it is truly a success or failure. No, his failure was something much bigger.
The Blood Elves needed magic to survive. But Kael'thas did not have the job of securing magic for his people, he had the job of securing their survival. When he became too focused on magic, he failed. This was when he left, betrayed, Illidan and joined the Burning Legion. That is just plain stupid. It was the Legion which created the Scourge which destroyed the Sunwell. It was the Legion that had driven the Orcs to their first devastating invasion of Azeroth. With them came the mobilization and renewed power of the trolls, again at the cost of Quel'thalas and the High Elves.
The Legion could offer magic and nothing else. The alliance with them cost the lives of many Blood Elves and caused massive disruption following the invasion of Silvermoon City. Draining demons may have been risky, but allying with them was insanity. Illidan may have been a bit crazy, but he was still powerful and was still loyal to Kael'thas, at least until the split.
Kael'thas lost sight of the goal: protection of the Blood Elves. Instead he focused only one one aspect: magic, and in the process stopped caring for the Blood Elves in favor of a smaller group of those who betrayed all life in service to the Legion.
What can we learn from this? Don't let methods replace goals. Don't let ends justify means. But also, don't ever truly die.
We did not call him the Ashbringer because of the sword. No, he was the Ashbringer without it, before it. He did not need to draw his weapon before turning thousands of enemies to ash. And yet, despite his power, it was all futile. The Scourge he could destroy, even the undead he could keep dead.
But there is one who springs from his ashes like a phoenix, and we will never forget that shrill cry... "merely a setback!"