I'm a mage main.
DK is my primary alt.
There is no comparison - the mage damage tier is leaps and bounds worse than the DK tiers.
I'm not saying DK tier75 and 90 are good. Far from it. I am saying that those tiers do not make the game less fun. The DK tier 90 actually is really cool, the only problem is when you compare it to classes like monk, warrior, priest, rogue, shaman, who all have exceptionally well designed (and dare I say fun) level 90 talents that cause a significant improvement in gameplay.
The tier 75 DK talents are just passive parts of how dk works. It's not good, it doesn't really add anything, but it doesn't force you to do stupid crap just to maintain your normal dps rotation.
Source: I have heroic progression kills on mage and DK from the current expansion (most of my DK heroic kills were from ToT, mage has heroic kills in all tiers).
I'm on the fence about the level 75 tier. They don't really add anything meaningful. But then again, I'd rather have something that is a meaningless passive, versus an additional mechanic that makes my life more miserable.