As a pally main tank focusing on Threat, you need to focus on a defense rating of 540. Next, focus on str and stamina. The more str you have, the more damage(threat) you will do, and the more damage you will be able to migrate. Strength is a pally's new best friend.
This spec focuses on 5 stacks of seals of the pure (15% increased holy damage), perfect for maximizing threat on single targets. This also drops some defensive talents like Devo aura, but I think its well worth the extra threat.
Level 80 Paladin (5/56/10) Holy (5 points) Seals of the Pure - Rank 5/5 Increases the damage done by your Seal of Righteousness, Seal of Vengeance and Seal of Corruption and their Judgement effects by 15%. Protection (56 points) Blessing of Kings - Rank 1/1 Places a Blessing on the friendly target, increasing total stats by 2% for 10 min. Players may only have one Blessing on them per Paladin at any one time. Improved Blessing of Kings - Rank 4/4 Increases the effectiveness of Blessing of Kings by an additional 8%. Divine Strength - Rank 5/5 Increases your total Strength by 15%. Anticipation - Rank 3/5 Increases your chance to dodge by 3%. Improved Righteous Fury - Rank 3/3 While Righteous Fury is active, all damage taken is reduced by 6%. Toughness - Rank 5/5 Increases your armor value from items by 10% and reduces the duration of all movement slowing effects by 30%. Blessing of Sanctuary - Rank 1/1 Places a Blessing on the friendly target, reducing damage taken from all sources by 3% for 10 min. In addition, when the target blocks, parries, or dodges a melee attack the target will gain 10 rage, 20 runic power, or 2% of maximum mana. Players may only have one Blessing on them per Paladin at any one time. Reckoning - Rank 5/5 Gives you a 10% chance after blocking or being hit by any damaging attack that the next 4 weapon swings within 8 sec will generate an additional attack. Sacred Duty - Rank 2/2 Increases your total Stamina by 6%, reduces the cooldown of your Divine Shield and Divine Protection spells by 60 sec and reduces the attack speed penalty by 100%. One-Handed Weapon Specialization - Rank 5/5 Increases all damage you deal when a one-handed melee weapon is equipped by 10%. Holy Shield - Rank 1/1 Increases chance to block by 30% for 10 sec and deals 211 Holy damage for each attack blocked while active. Each block expends a charge. 8 charges. Ardent Defender - Rank 5/5 When you have less than 35% health, all damage taken is reduced by 30%. Redoubt - Rank 3/3 Increases your block value by 30% and damaging melee and ranged attacks against you have a 10% chance to increase your chance to block by 30%. Lasts 10 sec or 5 blocks. Combat Expertise - Rank 3/3 Increases your expertise by 6, total Stamina and chance to critically hit by 6%. Touched by the Light - Rank 3/3 Increases your spell power by an amount equal to 30% of your Stamina and increases the amount healed by your critical heals by 30%. Avenger's Shield - Rank 1/1 Hurls a holy shield at the enemy, dealing [846 + 0.07 * SPH + 0.07 * AP] to [1034 + 0.07 * SPH + 0.07 * AP] Holy damage, Dazing them and then jumping to additional nearby enemies. Affects 3 total targets. Lasts 10 sec. Guarded by the Light - Rank 2/2 Reduces spell damage taken by 6% and reduces the mana cost of your Holy Shield, Avenger's Shield and Shield of Righteousness spells by 30%. Shield of the Templar - Rank 3/3 Reduces all damage taken by 3% and increases the damage of your Holy Shield, Avenger's Shield and Shield of Righteousness spells by 30%. Hammer of the Righteous - Rank 1/1 Hammer the current target and up to 2 additional nearby targets, causing 4 times your main hand damage per second as Holy damage. Retribution (10 points) Deflection - Rank 5/5 Increases your Parry chance by 5%. Improved Judgements - Rank 2/2 Decreases the cooldown of your Judgement spells by 2 sec. Heart of the Crusader - Rank 3/3 In addition to the normal effect, your Judgement spells will also increase the critical strike chance of all attacks made against that target by an additional 3%.