First, the obvious one. The sonic+light damage not affecting red-named bosses. It's obviously a design decision, since it's on the tooltip. I don't see what the difference between being able to take down a boss quickly and being able to clear a room full of trash quickly is. Since there are a lot of quests where the trash is negligible, it makes the epic moment a lot more useless in several quests/raids. Sure, the +40% damage and other effects still apply, but they aren't nearly as big. Red name immunities seem like they should just apply to things that trivialize the fight, mainly, CC abilities, instakills, and certain debuffs. Extra damage doesn't trivialize a boss fight any more than you can roflstomp trash with the same damage. No other damage ability (not counting helpless effects) are prevented in boss fights.
Second, the cooldown it creates when you use it makes it really hard to use in difficult content. Maybe not for a barbarian who just has to swing away, activate clickies now and then and twitch around to avoid as many hits as he can, but for most characters, not having access to your spells, potions, scrolls, and more (i.e., pretty much anything on your hotbars) for 10 seconds can make the whole ability not worth it. And if you use it 10 seconds before you rush into battle, letting the cooldown tick off before you make use of it, there's barely any time left to take advantage of it unless you're a bard with lots of song length enhancements.