Originally Posted by
Immortal
First bossess that had non-trivial mechanics and were tightly tuned, but still killable, were in Sunwell if I remember correctly. And since Ulduar (well, actually Sartharion was the 1st example) Blizzard started splitting normal and hardmode/heroic versions and tuning the latter tighter, while not expecting most of the raiding playerbase to kill them at least before the next content patch. After that some bosses were harder, some easier, but their diffuculty can be compared directly.