Guys, I'm getting pretty worried about Aspect of the Iron Hawk. And Spirit Bond, for that matter! They're bad talents. The passive damage reduction tied to Shadow Priests' Shadow Form is bad. Passive damage reduction for Moonkin is bad.
Greg Street has himself written many times about exactly the reasons these sorts of talents are bad, especially when they're a talent or glyph. They eliminate having an actual choice between compelling options and instead provide the player with one unequivocally correct choice and one or more "traps." These other choices may look good to the casual or the less than completely thorough player, but the vast majority of players will be picking the single, mathematically best option.
This problem isn't unique to passive boosts, of course! A Cataclysm Marksmanship hunter that didn't take Readiness, for example, is absolutely just speccing wrong. There's no wiggling around it: if you don't talent into that button, you've made a mistake. The difference is that at least with Readiness, when you spec correctly you get a new button to push that has exciting ramifications for how you plan out your burst phases in a boss encounter.
With the current Aspect of the Iron Hawk, you get nothing of the sort. You simply take that talent because it's the correct choice. Then 15% less damage happens to you. The end.
Compare this with the relationship between Exhilaration, Crouching Tiger, and the Glyph of Deterrence.
Picking these three choices feels really good. They all synergize with each other in fun and exciting ways. Crouching Tiger lets you push the Disengage and Deterrence buttons substantially more frequently, while Exhilaration and the Glyph of Deterrence improve survivability value of choosing to push those buttons frequently. Considered as a whole, this set of choices rewards and encourage you to look for opportunities to use those abilities skillfully. You're going to be actively looking for excuses to use Disengage, ways to abuse Deterrence, encounter mechanics that present puzzles you can solve as a hunter.
Aspect of the Iron Hawk does none of that, and yet we'll be forced to take it, because it will overall be more effective in getting guild-first kills.
Think about the thousands and thousands and thousands of damage that happen to you during essentially any progression encounter. There are rare exceptions like normal Baleroc, but for the most part there is plenty of unavoidable damage to keep the healers busy. That's part of how they punish you for taking avoidable damage: on a progression encounter, your healers are already going to be stressed, so making mistakes means you can die very easily.
Iron Hawk just shaves 15% off the top of that constant, ongoing damage. There is no way that the healing from Exhilaration and the DR from Deterrence will catch up to that. It's just impossible. Spirit Bond, in addition to itself being a dull and dreary passive talent, is a joke compared to Iron Hawk.
Iron Hawk needs to go. I am 100% behind Mr. Street's stated goal for Pandaria when it comes to compelling choice. I think the current talent model is looking good, and I think the philosophy is looking good in Diablo 3 as well. But we've still got a few of these reprobate abilities sitting around, and the last thing we need to do is add more of them.
Or well, that's my opinion anyway! If anyone can make a good argument in Iron Hawk's favor, I would be delighted (if somewhat incredulous!) to hear it.