Black Temple and Mount Hyjal nicely as my guild's tank, I am still trying to improve my style and playing elements. There's not much more I can do to build threat – the Devastates, Shield Slams, Revenges, Heroic Strikes, etc., are all going off at the right time. Gear is fine, I can pretty much tank anything in the game at this point and succeed. So why am I still dying? Why am I missing that critical moment when I could put up my Spell Reflect and live a second longer?
I think I've found it.
It's all about eye movement. Try this: focus your sight to the upper left hand corner of the screen, and now move your sight down to the bottom of the screen. If you have a large enough monitor, you completely lose focus of the text and items near the upper left corner. This is problematic for tanking in that the unit frames (those things that tell you who's in your group and who you're attacking) are by default located in the upper left corner, and the action bars are located at the bottom. So if you want to make sure you're going to hit something – or even look at your keyboard for a moment – you're moving your eyes quite a bit.