Sorry for the "late" reply, Arlan. Each type of stat has a value to a tank, and it's not always as simple as "X > Y > Z". For a Protection Paladin, here's how it breaks out, roughly. [Long post alert]
Defense Target: 490 (negates critical hits)
Avoidance Target: 102.4% (negates crushing blows)
Avoidance formula: 5%* + [Defense Miss %] + [Dodge %] + [Parry %] + [Shield Block %]
* I always thought this was 1.4%, but EJ says otherwise.
For "pure" avoidance, leave Block out of the above equation. This comes into play in the following situations: against mobs that deal elemental melee damage (Hydross, etc.); against special abilities that aren't considered "resisted" when you block them.
Holy Shield provides 30% Block when active. Against a single opponent you should seldom run out of charges, but due to the cooldown and latency there's an unavoidable interval between when it fades and refreshes in which you can get crushed. Include Holy Shield (plus the badge Libram) in your avoidance calculations.
Redoubt provides 30% Block when active, but its random nature means you cannot rely on it to become crush-immune. Ironically, it is most likely to be active when AoE tanking, and mob packs are not boss-level and don't crush. Do not include Redoubt in your avoidance calculations.
For general purposes, the formula you should follow is: Defense to target, then Avoidance to target (using Shield Block as much as possible), then get your Stamina up. Don't be afraid to use Warrior tank gear but remember that you need spell damage to generate threat. (Paladin tanking gear is well supplied with spell damage.) As you graduate to better gear, you will replace some of your Shield Block with Dodge; as long as you stay avoidance capped, Dodge is generally superior. Just don't go too crazy; a lot of Paladin threat generation is block-based.
Advantages: applies to everything: Miss, Dodge, Parry, Block, as well as reducing critical hits. It's the only stat that directly affects enemy Miss chance.
Disadvantages: after the 490 target to be crit-immune, you get slightly more value per point from individual stats rather than from Defense as a whole.
Advantages: better rating ratio than Parry, effective when disarmed
Disadvantages: none; it's the best Avoidance stat
Advantages: your next attack after a parry is faster, improving threat generation
Disadvantages: more expensive than Dodge, cannot use when disarmed
Advantages: more than twice as cheap as Dodge, making it the ideal stat to reach your avoidance cap; also improves threat generation due to Holy Shield and Blessing of Sanctuary procs
Disadvantages: blocked attacks still deal damage, so it is not a "true" avoidance stat. Worthless against mobs that deal elemental melee damage instead of physical.
Advantages: improves mitigation for blocked attacks. For an avoidance-capped tank, all attacks that do not miss and are not dodged or parried should be blocked.
Disadvantages: Paladins gain no threat generation benefits from SBV, unlike Warriors. Worthless against mobs that deal elemental melee damage.
Advantages: increases threat generation
Disadvantages: doesn't keep you from dying
Note: 300 is a good number for pre-Kara tanking; 400 for Prince/Gruul/Mag; and 500+ for SSC/TK/ZA. This is in your base gear.
Hit Rating (Cap: 8.6%)
Advantages: increases threat generation by making your melee attacks more likely to land (and by extension makes Seals more likely to proc). Affects Righteous Defense (taunt), Avenger's Shield, Hammmer of Wrath, Seal of Blood (reportedly).
Disadvantages: lots of Paladin abilities are considered spells
Spell Hit Rating (Cap: 16%)
Advantages: increases threat generation by making your spells more likely to land. Affects Consecration, most Seals, all damaging Judgements, Exorcism, Holy Wrath, Holy Shield, Retribution Aura, and Blessing of Sanctuary.
Disadvantages: doesn't help melee attacks hit (see above)
Advantages: keeps you from dying; damage spikes are what kill most tanks, so the more Stamina you have, the better
Disadvantages: doesn't help you generate threat or avoid attacks that have undesirable status effects
Advantages: sustained threat generation
Disadvantages: when you are MT, you get most of your mana from being healed anyway
So, what does all this mean? You need different sets of stats for different situations.
AoE tanking: Spell Damage and Block % are key. The more you block, the more threat you generate, but Consecration is still your main AoE threat ability. You need a decent amount of Stamina, but not as much as a MT. Shield Block Value improves mitigation and reduces the strain on your healers.
MT melee boss: Must be crit and crush immune. Stamina and avoidance are your secondary goals. Against traditional tank and spank bosses, you'll have no trouble generating threat unless you go overboard on Dodge/Parry or have critically low spell damage. In general, the faster the boss hits (not harder), the more threat you generate.
Off-tanking: Spell damage and Intellect are needed to be able to keep up with threat generation. You won't be taking as many hits, so you will have mana issues. Keep a stack of pots on hand for this situation. Depending on the fight, though, you may be tanking the boss yourself or eating secondary attacks, so you may just use your MT set unless it's very low on Intellect.
Elemental boss: This one is hurty for Paladins. Fortunately, they can't crush. Unfortunately, you can't block their attacks, so you lose a major component of threat generation - plus, your core mitigation is not as good as a Warrior's. Stack health, as much non-block avoidance as possible while staying crit-immune, and whatever resistance stat may be required.