Tamral, I think your approach to tanking is exceptional. It is good to over analyze things as a tank - skill is hugely important, but if wearing that ring in your bag actually is better, and the only reason you're not wearing it is because you're too lazy to do the math, then really, shame on you as a tank - and it is certainly not going to help your progress.
Note, that is not meant to be a knock on anyone in these forums... just saying great post by Tamral.
As to Ihmes.
Is tanking hard?
No not really - but neither is any class. But just because it isn't hard, doesn't mean that everyone performs at even remotely the same level.
Tanking is probably one of the more intricate roles, as mistakes here are much more likely than any other single individual to wipe a raid.
Being good at this game generally transcends your class.
Being good involves awareness:
1. Knowing what abilities to use, and why.
2. Knowing what is happening
2a. What the boss is doing, why, and when, and how often, and what/when to expect more.
2b. Knowing what you're doing (sounds weird, but this is probably the most key point - movement and adjustment!)
2c. Knowing what OTHERS are doing. Many top-end guilds will readily accept raid leaders and guild leaders of other guilds who apply simply because if you can manage others, you can likely manage yourself. Doing well as a raid member involves managing your own contribution, and also understanding the motions and actions of other players... A good healer for instance, MUST KNOW when to compensate for a loss or adjustment! Tanks are no different.
3. Just being alert, not afk, all that jazz.
Probably 4-10 that I'm forgetting now.
But the actual skill of pressing 544542545422444444544522534424443354254434444444425422244444445444452 (4 is clearly HS) is not that great. Adjusting, awareness, knowledge is. MT's should know what they're doing. They have a huge impact on the raid.
And yes, there are good tanks and bad tanks. There are even exceptional tanks. As a raid leader, especially one who had to deal with 8 tanks of different quality on 4 Horsemen - you can tell immediately every level from lets say a rank of 0 to a rank of 100 (Gamespot scale here, Eidos makes it so I cannot score below 70 though). There really is a lot that personal skill can contribute to the class.
PS: I agree gui and macros are important. A great tank can become even better if he/she customizes their gui in a way that matches their own personal preference - if well done you can consolidate information and act more efficiently. Yes, the default gui is sufficient; but just because you do something and it works, doesn't mean it is the best way to go about it. I think about half of the demotivators address this issue - just picking one randomly:
http://images.despair.com/products/d.../tradition.jpg
And we know if it is on despair.com, it is fact.