A paladin can tank with the best of them. However, before you log into World of Warcraft, make sure you know the basics of tanking.
The Paladin uses mana where the other tanks use rage. While this originally prevented them from being viable tanks for long fights, mana regen from healing received (via Spiritual Attunement) makes this no longer an issue. Tanking Paladins affectionately refer to their “Blue Rage bar”, as SA replenishes their mana supply so long as they are getting heals. The remaining functional differences are twofold: first, a paladin has a much larger (and scalable) functional resource pool (i.e. 100 rage is worth X seconds of ability use, 4000 mana will be more time of ability use than X), offset by a greater need for efficient spending. Second, they start a fight at full resource, making heavy frontloading possible.
The Paladin generates threat through Holy spell damage (with threat multipliers). Their overall threat model is more like a Druid than a warrior: high damage with bonus threat for damage dealt. They differ from Druids in that their damage source is unaffected by any mitigation mode in the game. Holy Damage has no resistance value: the spell damage can “miss”, resulting in a full resist, but partial resist or school-specific resists don’t apply. Since the damage is magical, rather than physical, the armor of the mob has no impact on the threat generated by the paladin. While some of the threat is generated from holy damage procs on successful weapon swings, a lot of Paladin threat is coming from reactive procs (i.e. damage when hit) and AoE DoT, so even outright mob avoidance has only a moderate effect on threat generation. This spell-based threat mechanic does make the paladin less effective on mobs that silence (see below) and are magic immune (though resistances aren’t a factor).
The Paladin generates threat through multiple sources that are active simultaneously, most of which are fire-and-forget. At any point in a fight a Paladin will probably be building threat from seals on weapon swings, hits taken, area of effect damage, hits blocked (at all times), bonus damage on hits blocked (when Holy Shield is active). These multiple damage sources all scale with gear and add up FAST. In any multi-mob encounter the Paladins screen will be covered with a constant stream of numbers.
Since much of the damage and threat of the Paladin is passive or nontargetted, there is no real limit to the number of mobs a Paladin can hold. The threat built on non-targeted mobs is substantial: instance nonelites will usually kill themselves from reactive damage long before DPS gets to them on the target list, and elites will still be substantially weakened. Paladins rarely require crowd control: the only limit to the number of mobs they can tank is the amount of damage the healers can stay on top of.
Paladins can frontload a LOT of threat. At level 70 (in decent pre-raid gear) Avenger’s Shield (the primary pulling ability) will generate 2-5k threat on the three targets it hits, followed by 1-2k threat from Judgement of Righteousness. While applying 3,000 to 7,000 threat on the first DPS target before it gets into melee range is remarkable, consider the real trump card on Paladin threat generation: Avenging Wrath ups all damage dealt (and thus all threat) by 30% for 20 seconds, once every 3 minutes.
Paladins are somewhat limited in their “Oh No!” buttons. While Righteous Defense does function through a Divine Shield (i.e. the paladin can bubble, then taunt, and the mob will come back and smack at them to no effect), the timing on this is very tricky and only works on mobs that can be taunted. Ardent Defender (always active) drastically reduces all damage received if the Paladin is below 35% health, a very poor man’s Shield Wall: an attack that takes them below 35% may be followed up an attack that flat out kills them, even with the damage reduction.
A Tankadin who is not tanking is more or less dead in the water. Paladin tanks have very small mana pools: 5-6k at 70 pre-raid, at best. They rely on mana regained from Spiritual Attunement so that they can focus their gear on mitigation, and as such don’t have much staying power when they aren’t a primary healing target. They miss most of the mana efficiency options from the Holy tree so their healing is limited. Their best DPS options require taking hits to function, so their damage is laughable (substantially less than a Protection warrior) when they’re not tanking. With a gear swap they can be a support healer (as well as handle buffing and decursing), but they are limited when it comes to changing roles mid-combat.
The single greatest challenge a Paladin will face is their own versatility. A Paladin has more manual control over their pacing during a fight than other tanks, making efficiency very important. They have more things to keep track of, so a lot of multitasking is required (even on single-mob fights). They have a wide range of options that may be required at various stages in the fight, and it’s up to the Paladin to determine when to emphasize threat, mitigation, or efficiency.