Some people seem to think that being a tank is about having the right kind of gear and having the right kind of spec. That isn't what tanking is about at all. Tanking is about being able to take The Damage and keep The Aggro. As long as you can do that you count as a tank, no matter what gear or spec or class you happen to have. This is the reason some classes that usually never go as tanks can tank on occassion, like hunters, warlocks and shamans.
Of course at endgame taking damage and keeping aggro often equals having a certain spec and gear, which is why people confuse the two. Later on the tank trees will offer a set of damage reduction skills that will be necessary to survive. But until then, you are free to experiment and might in fact find better solutions than if you had gone the conventional way.
One such solution is the Retridin Tank. Here are six good reason why a retridin makes a better tank than the conventional prot-spec.
1. Double the threat
At level 20, retridins can spec into Seal of Command. Since they changed it, it now hits two targets instead of one, effectively making it like a glyphed maul or sunder armor. But it's better since it hits two targets on every swing, and not only when using a certain skill. Awesome to keep aggro on multiple mobs without having to spend all mana on consecration all the time.
2. Never oom
At the same level, 20, you can glyph Seal of Command. That means you will get 8% of your base mana each time you use a judgement, and if you combine that with judgement of wisdom one of the biggest problem of being a lowbie protadin - constantly ooming - has been solved. When tanking as a retridin you'll only have to drink after dying.
3. Less mana breaks = faster instancing
One of the biggest issues of lowbie tanking overall isn't just that you might oom, but that every other caster in your group does. With talents like Judgement of the Wise this will be way less of an issue since everytime you judge you'll replenish everyone in the party, this will also increase your own mana regen even further.
4. Ranged cc
As a protadin you have no cc whatsoever and will have to rely on your party members to take care of it. And we all know rule #1 of tanking is to never rely on anyone to do anything outside the basics. As a prot you'll have one tool only (before level 50) to keep aggro on ranged mobs, Hand of Reckoning. As a retridin you'll also be able to cc ranged mobs when needed. Any tank loves to be able to cc.
5. Instants R us
Speaking of ranged threat, with Art of War you'll be able to use instant exorcisms to get threat on mobs that are too far away, or as a strong pulling tool. Any way you care to use it, you'll love the fact that you have it, since not only does it great threat, it also does great damage. And if things go really wrong you can use your Art of War procs to cast instant heals on yourself. What tank wouldn't love to be able to do that?
6. All the little things
There are several other talents in the ret-tree that are good for tanking, such as Vindication which lowers the attack powers of mobs (and thus the damage you take) and Pursuit of Justice which increases your run speed. Perfect for catching up with whatever it is that you need to catch up with, but as a tank that will be needed more often that you think. Also Divine Storm is about as good as Hammer of Justice since it hits the same amount of targets, but also heals (but deals a little less damage with a onehander). Oh and yeah, Deflection is in the ret-tree. Many of these talents are such that prot-paladins pick once they're done in the protection tree.
So if you intend to level a paladin, I definitely recommend going ret. That means you'll be able to quest in a good pace, and you only have to grab a shield to make a good tank (although preferrably you get other pieces that suit tanking as well, like something with stamina). I'm guessing a ret-tank has no problem tanking instances at all up until level 60, and perhaps not even until level 70, before prot actually gets the upper hand with all the damage reduction talents that will be more and more needed.