you get 540 defence.. this is very important, becuase with 540 defence you cannot be critted by any boss in the game
next you stack stamina, then when you have a nice HP pool (around 26-7k) you should start focussing on Avoidance, this is Block/Parry/Dogde, IMO Dodge is the greatest avoidance stat atm, bosses are hitting big, not small, so block is not that useful as it only stops a bit of the damage, but when you dodge you take NO damage, although, having block is good for things like trash.
You should also be using Seal of Vengance, the one that stacks a DOT up too 5 stacks, and then also use the glyph that will give you 10 expertise when you have Seal of Vengance on, this seal is the best for threat. its also becuase of this glyph you will not need too stack expertise like a warrior does.
I hear alot of people saying you should use a 969696 rotation, i personly dont do this, i always start with Shield of ritousness, then Hammer of ritousness for burst threat from the start, i then use judgement ( Not the best for threat, but i have taken the ret talent that gives 3% crit too raid when judge is up as we dont raid with a ret )
after that choose whatever skill would be best for the situation.. after that skill my ShieldoR will be back off CD and after that HoR again, (If you want too keep your threat high, use Shield of Ritiousness on EVERY CD
You also have a skill called Divine Protestion, it is like your bubble, but instead of making you immune, it makes everything do 50% less damage for the time its up, this is brilliant becuase when you use it you lose NO threat, unlike bubble. You should use this skill at a time your going too take alot of damage, or notice alot of people taking damage.. it will give your healer them presious few seconds too heal everyone up and still keep you alive, using this skill at the right time can be the difference between Wipe and Clear
well, i could go on for ages talking about prot palas, but if you have anymore questions ill be happy too help.