10-7-2009: I wrote the original article almost a year ago, and it was in sore need of an update. I've now edited this post to bring it up to date.
The first and most important cap you want to reach in your gear is being uncrittable, which comes from Defense. At level 80 you need 540 Defense which equals out to about 689 Defense Rating. To quote Neo, "Whoah". It's a metric ton of defense needed. It is something you will need to actively work towards in your gear, but it is completely attainable with crafted items and rep rewards.
Edit 11/21: Petrus of Gorgonnash has put together a tidy list of gear that will get you to the Crit cap without relying on a single dungeon drop. You can see his list here: http://www.failsafedesign.com/maintankadin/viewtopic.php?t=17158. Well done, sir!
After you are Uncrittable, you should start working towards the The Block Cap. This may be a new concept for Paladin tanks, but it really isn't. The Block Cap is what used to be known as Uncrushable, i.e. a combined Miss, Parry, Dodge, and Block of 102.4%. Once you reach that point, any additional Block Rating falls off the table. You are 'capped' on the amount of Block you can have. It should be noted that since Holy Shield was changed to have its duration be longer than it's cooldown, it is now safe to assume 100% uptime on Holy Shield when calculating your Block Cap. Holy Shield is 30% chance to block.
Being at the Block Cap is very helpful because it means that every unavoided attack from the Boss will be a Block. Paladins are the only class now that can stay Block Capped 100% of the time. Each Block takes off damage equal to your Block Value. If the Boss hits for 20,000 and you have 2000 Block Value (not unreasonable), you effectively shave off 10% of the damage each and every hit. That can add up to a huge amount of damage reduction over the course of the fight.
It also provides Paladins with a predictable damage intake, and makes life easier for your healers.
The Block Cap is especially effective for tanking adds on Anub’arak.
So you can't be Crit, and every incoming attack is a Block at the worse case, what's next?
Paladins are much like Warriors and we want Hit on our tanking gear. Math has shown that Hit is either the 2nd or 3rd best stat to increase threat depending on how much Strength you have.
Hit is what is checked to see if your Taunt resists. While your melee swings need 8% hit to connect, taunts are checked against spell hit, and require 17%. There is basically no reasonable set of gear that will get you to 17%, sorry.
The Glyph of Righteous Defense should help to eliminate most HoR and RD resists, and I suggest you take it. Paladins do have two taunts, Hand of Reckoning and Righteous Defense. The chance that both miss is small, however, it can still happen.
You need 8% to hit to be Hit capped. That translates to 263 Hit rating at level 80. If you can wrangle one of our Draenai friends into your group, you will need only 7% hit, or 230 rating.
You can't miss, but the boss can still Dodge and Parry you.
Expertise is what stops the Boss from Dodging and Parrying. At 0 expertise, the boss has a 6.5% chance of dodging you, and a 14% chance to parry you.
Each point of expertise reduces the chance the boss will dodge you by 0.25% as well as the chance he'll parry you by 0.25%. Each point of expertise after 26 is half as effective as it was below 26, because the boss already has a 0% chance to dodge. Thus, expertise gets weaker above the dodge soft cap. It's entirely useless above the parry cap of 56, but this is rarely achieved in practice anyway.
Expertise affect Hammer of the Righteous and is in fact the only way to get rid of the occasional ‘Deflect’ message, which most theorycrafters seem to think is a parry of a ranged attack.
Expertise has also shown to be an excellent stat for reducing damage. Credit goes to Math genius Theck on Maintankadin. You can read the proof in this thread http://maintankadin.failsafedesign.com/forum/index.php?f=6&t=25611&rb_v=viewtopic.
From the numbers in the latest stat analysis, expertise is 69% the threat of STR below the dodge cap (19.83/28.81), 34% above the dodge cap but below the parry cap (half of it's value below dodge cap), and 0% above the parry cap.
The avoidance value of expertise varies a lot, with multiple factors (swing speed, boss swing speed, avoidance, etc). I'm not sure whether I'd feel comfortable giving values quite as specific as you have. It might be fairer to say that it's about 40-70% as effective as dodge rating, and 2-3x more effective at reducing spikes, provided the boss has parry-haste enabled.
You can get 6 expertise from talents and 10 from the Glyph of Seal of Vengeance (skill not rating mind you). 26 Expertise is considered the Dodge cap. At 26 Expertise, the Boss can no longer Dodge your attacks. You need 56 Expertise to hit the Parry cap where the Boss can no longer Parry you.
Humans and Dwarves can get another 3 to 5 points depending on the weapon they are using. Humans get 3 Expertise with Maces and Swords. Dwarves get 5 Expertise with Maces.