BC stands for "Burning Crusade", not "The Reign of Illidan in Outland!". So it really only makes since that a Burning Legion Lord would be the last boss of the expansion whether it was a mistake on Blizz's part or not.
Both were the end of WoW. </sarcasm>
Even though so many people are hating on halion, he is probably my favorite fight so far, and one of the few fights where I enjoyed the mechanics so much that I wanted to go back just to do them. However, it is obviously lacking in lore/substance. Either way, I find the mechanics to be more fun than LK, or any other fight in ICC to be honest.