First off, today we'll focus on the basics. Veteran tanks won't need all this information, but you're all free to read and nitpick because, well, I'm your whipping boy. Ok, so, you want to tank? The good news is that ever since 3.0, Paladin tanks share pretty much all gear with the other plate-wearing tanks. No more of that silly Spell Power nonsense. You're looking at gear with Stamina, Strength, and all those tanking stats like Defense, Block Rating, etc. This means it's easier to gear up. Actually, you should probably think about gearing up now before dual specs roll around and that Holy Pally rolls on your Red Sword of Courage for his "other spec". For today, let's take a look at some attributes that contribute to our survivability.
Armor is the most basic form of damage mitigation in the game. It mitigates Physical damage and the more of it you have, the better. Fortunately for Paladins, they wear plate, which naturally provides the most mitigation. Armor damage reduction has a lot of math and since we've got a long discussion ahead of us, we'll skip that. You can always just look at your character sheet in-game to see how much damage you're staving off.
Furthermore, there are a lot of armor-increasing buffs in the game, so this will be one of the things you don't have to actively think about. It should be worth noting that shields have an extremely high amount of armor compared to other pieces in the game so your armor will jump up considerably with a good shield. Armor does nothing to mitigate Magical damage.
Stamina is arguably the most important ability score for any tank, but it's extra special for Paladins because it translates directly to Spell Power with Touched by the Light. I know I said you don't have to look for Spell Power plate anymore, but it still figures into a lot of tank abilities. In fact, Tankadins deal Holy damage from more sources than any other kind of Paladin -- even Retadins.
This is why Stamina is king. The more Stamina you have, the more Spell Power you have, and consequently the more damage and threat you generate (besides, it's awesome to top the meters on massive trash pulls). Needless to say, the more Stamina you have, the more health you have, and the longer you'll last against big bad bosses. Fortunately, you can and should increase your Stamina with five talent points in Sacred Duty and Combat Expertise for a total of 14% increase. Once a problem area for Paladin tanks, high health now one of your best assets.
The next best ability score for a Tankadin is Strength. Each point of Strength translates directly to two points of Attack Power, which contributes to white damage. It's cool but not such a big deal. However, Strength now also contributes to Block Value, and that's a big deal. The more Strength you have, the more damage your blocks will absorb and more damage you'll deal with Shield of Righteousness. You can raise your Strength by 15% with the first tier Protection talent Divine Strength.
None of the other stats matter too much after that. Agility, for example, contributes to Dodge and Crit, but you're better off getting those raw attributes instead of Agi because it takes about 52 Agility for every 1% of either. Intellect doesn't matter as your mana pool won't be a problem -- even with 0 Intellect in all your gear -- because of Blessing of Sanctuary, Divine Plea, and the mana received from Spiritual Attunement. Spirit doesn't matter at all.