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.
This can be reconfigured quite easily however. I use X Perl for my unit frames, so I just dragged the targets down towards the center of the screen. I did the same with my action bars with Bongos – stacked them up right below the unit frames. Click on the image at the top of my post for a high resolution version of how I have things setup.
Now when something is happening, all I have to do is move my eyes around the center of the screen. This is a very short space that I need to focus on, and thus while I can't focus on everything at once, I can at least see the color changes and notice that something is happening much quicker with my information centralized.
This might not be a big thing to those of you who already have your UI set up like this. I'm sure I'm going to get a bunch of "you noob" comments and what not. But I'll point out that I've been successfully tanking everything for years like this. And while I'm not the best tank in the world, I never seem to have a problem finding a group – or being bugged for one (kidding friends...you know I love you all.)
Since making these modifications, I'm using Last Stand 42% more often, Shield Wall 74% more often, and various health potions/stones a little over 10% more often. That's all well and good, however my Spell Reflect is up a whopping 210% in use. One night I used it 88 times. If you figure each spell does on average 4,000 damage to me, that's 352,000 less healing needed on me, and that much more threat done by me. That is quite a number.
What other tips might you have to reduce your physical movement while tanking? Share and be heard!