There are four essential tanking stats, or groups of stats:
Of these, the top three are the most important. They are what separate a "tank" class from a dps class. After all, there's a reason we don't let the mages tank. While Threat is important, a tank is the class which can deal with incoming damage in the best manner.
Initially, Avoidance is the most important of the top three stats. Avoidance is the route to eliminating Criticals and Crushing Blows. However, once Criticals and Crushing Blows have been eliminated, Avoidance becomes much less vital.
This is because pure Avoidance (miss/dodge/parry) is probability-based. Raid fights are long, with many attacks. It is highly probable that a tank will take an unlucky streak of hits at some point during the fight. A tank needs to be prepared for the worst case scenario. Unless you have 100% pure Avoidance, Murphy's Law will inevitably bite you.
Now looking at these three categories, it immediately jumps out that--unless something significantly changes--paladin tanking will always be an inferior option for raid boss tanking.
Why? Because paladins are inferior to warriors in all three important stat categories.
For Avoidance, warriors have Shield Block, which allows them to eliminate Crushing Blows much faster than paladins can with Holy Shield. This also has the additional effect of allowing a warrior to reduce the importance of Avoidance earlier, and get an additional lead in the other two important categories.
For Mitigation, warriors have 10% melee mitigation from Defensive Stance, versus 6% from Improved Righteous Fury. They have 16% spell mitigation, while a paladin only has 10%. Finally, because so much of a paladin's Avoidance is gear-based, a paladin loses a significant amount of Avoidance when switching into resistance gear. A warrior loses far less Avoidance because her Avoidance is an innate ability.
For Health, a warrior simply has more base Health than a paladin. Additionally, while the paladin is still gearing for Avoidance, a warrior tank can start gearing for Health, and increase her lead over the paladin.
Even Threat is not a clear-cut victory for the paladin. The paladin is a clear winner in AoE Threat, but warriors can actually put out a significant amount of single target threat. I am not yet willing to judge that paladin single-target threat is better than for warriors. For AoE or front-loaded threat, paladin threat is superior.
It is interesting to compare druid tanking with warrior tanking. Druids are clearly inferior in Avoidance, as they cannot parry or block. But they have superior Health. And Mitigation is close to a draw, as druids do have much higher armor, but no innate Defensive stance. Because druids are actually competative with warriors in two of the three categories, they are a reasonable alternative to warriors.
There is one situation where paladin tanking can be superior to warrior tanking. A fast attacking boss can eliminate the warrior's Avoidance advantage, giving the paladin superior Avoidance for a portion of the fight. However a warrior still has superior Mitigation and Health.
Finally, paladins require a fifth stat: Mana. Unlike warriors and druids, paladins need a stat unrelated to the true tanking stat categories to fuel their offensive and defensive abilities. Because of this, paladin tank gear will always be inferior to warrior tank gear as some of the total item budget must go to the fifth stat, reducing the amount available for the true tanking stat categories.
It is my conclusion that--for cutting edge content--paladins will always be sub-optimal tanks compared to warriors and druids. Paladins are inferior to warriors in the three most important stat categories, at best have a slight advantage in the fourth category, and--unlike the other tanks--require a fifth stat category in order to function.