The following was a response of mine to a topic at Maintankadin forums.
What is required to make Paladins an equally attractive tanking option?
-There's virtually no way you aren't going to lead against the Warrior in Avoidance and "uncrushability." The person judging your status as a tank needs to understand this.
-Paladin Threat scales well (so does Warrior Threat, thanks to +Hit and SBV). It is understood that you can gain a great deal of Threat by adding +Spell Damage to your gear; however, it is also true that Threat production in raids has not been the primary purpose of the Paladin due to the Off-Tank role many assume, be it by prejudice or practicality. This allows flexibility in gearing to the point where most Paladins do not need +Threat anywhere but on their weapon.
In addition, both the Paladin and the Warrior are capable of producing more Threat than standard gearing allows. This is due to the ability to switch separate pieces of gear in. As you are keenly aware, Paladin Threat in 5-mans scales far better than Warrior Threat (yay, Rage); in raids, both classes keep in pace.
To prove yourself as a tank, it may be helpful to fine tune some of your gear for situations where you are asked to tank initially. Do so in such a way that you can demonstrate very high Threat-Per-Second output -- higher than normal -- and win over the DPS'ers as well. There are many Warrior tanks who do not fully understand how to maximize their own Threat, so if you show the skill, that should draw considerable attention to the possibility of Main Tanking.
-The major balance issue comes in Effective Health. If you can match or exceed the Effective Health of a Warrior, you are equally suited -- if not a bit more suited -- to tanking a good deal of content. Granted, not all of it, because there are some unfair restrictions in the form of Silence gimmicks, etc.
Let's assume for a moment that reaching "uncrushability" causes a disparity in Karazhan. Let's further assume that many new raiding guilds -- most of which are the ones that Paladin Main Tanks are going to be popping up in -- are going to determine their prejudices in Karazhan and Gruul's Lair.
The disparity in Karazhan is such that a Warrior can more easily obtain a higher Effective Health -- that is, the combination of Stamina, Armor, and Shield Block Value that determines the amount of raw damage a player can survive. A Warrior may come out in this gearing process with much more survivability, simply because it is easier for him or her to reach uncrushability. However, as Paladins are aware, the Warrior uncrushability is not absolute, and has a weaker foundation than Redoubt.
Finally, let's make clear that the Warrior and Paladin meet together in terms of later raid content. If the Paladin can maintain a tanking position in later raid instances (SSC, TK, etc), he or she will reach all of the Effective Health minimums of the Warrior. Conversely, the Warrior will be gaining the absolute and reliable uncrushability of the Paladin as his or her Avoidance rises. Short of gear discrepancies and gimmick fights on the part of Blizzard, there is no reason a Paladin cannot Main Tank every fight in World of Warcraft.
Now, let's backtrack. The problem comes out of the realm of theorycraft in Karazhan. The Warrior is not absolutely uncrushable, but he or she appears to the raid to survive longer and last through bigger hits. Healers feel they are having an easier time with the Warrior because -- when he or she isn't being crushed -- the Warrior is soaking up damage. Nevermind that the Warrior is also occasionally dying outright to "perfect storms" of damage and crushing blows, the raid is willing to accept that occasional wipe as they do every wipe... pick it up and try again (and try to pass blame off on a healer, of course).
All of this is just my viewpoint. Assuming I was mostly accurate on this -- and please correct me if I wasn't -- I would simply give this solution.
To be equally attractive to Warrior tanks, do everything in your Holy power to get more Armor, Stamina, and Block Value.
Do not sacrifice your uncrushability, but go above and beyond in gearing when in Karazhan or any early tanking. Yeah, you have to work harder than Warriors, but that's not to say the good Warriors aren't passionately doing the same (ideally too many of those aren't in your guild!).
For the record, nearly every e-mail or PM I've received has pretty much left me with a better impression of Paladins as tanks. Again and again I am finding Paladins who have considerably better Effective Health than lazy Warriors in their guild. This morning, I was shocked to compare armorys of two guildmates on a server in the EU -- the Paladin had 10% more Effective Health (factoring in Defensive Stance!) than the Warrior. This, on top of having better Avoidance and uncrushability left me a bit at a loss as to why the Warrior was being chosen.
Another poster here brought up the question, paraphrasing, "If we stack Armor, won't we be like Druids?" No. This is due specifically to the relationships that produce Effective Health. Druids -- while good tanks in their own right -- require an additional 50% Effective Health to be as reliable as tanks as Paladins or Druids. In other words, the Armor they have is the only way they can reliably overcome the crushing blows that Warriors or Paladins don't have to worry about as much.
Anyway, here's a link that might be of interest on this discussion:
There is a calculator on the right side which can help determine your Effective Health, though I haven't added anything for the (6% is it?) damage reduction of a Paladin.
Thanks, and tank well,