Basically, there are two competing theories as to the best statistics weighting for paladin tanks, and lots of math to back each side up. The two camps are the Avoidance camp and the Mitigation camp.
Avoidance tanking is the method we're used to from Cataclysm; stack dodge and parry. We can no longer reach total combat table coverage, but the idea is that a completely avoided attack is a good thing so more of that is more than a good thing. If you're going for avoidance you'll find that you end up with way more parry than dodge. However, this is a good thing because the diminishing returns on dodge kick in way earlier than the diminishing returns on parry. Theck wrote a macro you can run to figure out where your parry-to-dodge ratio currently stands.
" /run ChatFrame1:AddMessage(format("Ideal pre-dr parry/dodge ratio: 3.597, yours is %.3f%%, if too high reforge more parry to dodge",(GetParryChance()-3.67)/(GetDodgeChance()-5.01))) "
Then there's Mitigation. This abandons dodge/parry stacking and has you reforge like mad. The most important thing to cap is hit, followed by capping expertise, then go heavily into mastery and haste. I know, I know, it sounds like a dps gearing scheme. I mean, haste? For a tank?
See, the current rotation is pull with a Judge, CS, Avenger's shield, something, judge, Shield of the Righteous, CS, something, unless Avenger's Shield procs again and your rotation kinda goes out the window. But the main idea is to mash CS and Judge whenever they're off cooldown and hit SotR as often as possible.
Hit and expertise means that you don't have misses with CS, so you build up your holy power at a smooth rate. And haste means that you're waiting less time for that judge and CS to come off of cooldown and give you that SotR or 3-HP WoG that you so desperately need.
Hit cap is 7.5%, or 2550 total hit rating. Expertise 'soft' cap is also 7.5% or 2550 total rating - this means that a boss won't dodge your shots. Expertise 'hard' cap is 15%.
Don't worry, we're still supposed to avoid crit. Tanking isn't about flashy critical hits, it's about control. Seriousface.
I haven't done much reforging yet because I've still got levelling greens and we haven't yet set foot in a raid. But I'm planning to try reforging and gemming to the Mitigation scheme once we do start active raiding, to see whether it makes me any less squishy.