With alot of the stuff in this thread, I'd have to add something: Sense of progression.
We play this game to have fun and feel entertained. When we're trying to kill a new heroic boss, the fun and entertainment comes from the sense of progression. If you feel you're getting a little bit further or getting a little bit better at dealing with fight mechanics (Lei Shen), you feel a sense of progression, and thus the fight is entertaining and fun. If you continiously wipe at the same spot, or because of the same mechanics, no matter how you try to deal with it (Dark Animus), everyone gets tired and wants to do something else, because you're not progressing, and thus you're not having fun or feeling entertained.