Prot Spec? Why bother.... For most of the game anyways.

Frankly, I think Ret TPS and mitigation while tanking would be inferior to a prot paladin. You lose most the significant holy damage and several mitigation talents.

If a ret paladin spends more than 20 points in prot, she does not get divine storm. So let's assume she must spec 0/20/51 (in order to get the most "tank" talents).

-no less avoidance
-Cannot use improved devotion without losing Sanctified retribution (+2% tps)
-Can maximally use 2/3 Devotion due to point restraints and the necessity of Improved Righteous Fury and Toughness.
-Loss of holy shield
-Loss of Sanctuary
-Loss of Ardent Defender
-Loss of ~12% stamina (more with kings)
-Loss of 6% expertise
-Loss of 3% mitigation
-Loss of 6 spell mitigation
-Loss of Thunderjudge (applicable by others)
-Loss of 30% BV
-Loss of 5% seal and judgment damage (assuming SoV)
-Loss of Sacred Shield efficiency (assuming it works with Touched by the Light)

So what does ret gain over protection? Assume the ret is in prot gear, and compare to the standard 5/60/6:
-8% run speed (7% higher than tuskarr's)
-Disarm duration reduction
-5% critical strike chance (over prot's 6%)
-25% Judgment critical strike chance
-Possibly "50% DR bypass" during a lower-CD avenging wrath.
-A DoT based on crits
-A CC for trash
-2% or 5% DPS (vengeance/crusader vs 1H Spec)

Conclusions for threat:
The ret paladin loses in threat unless the bonus crits and 1/4% DPS, 5% seal/judge damage, 6 expertise, and 30% BV on ShoR. Based on the math given in other threads, this won't happen based on the 6 expertise alone.

In the case of avenging wrath, almost all of a protection paladin's threat bypasses DR since it is holy damage. Thus, having 50% of a ret paladin's damage bypass DR simply puts him closer to par with protection's already significant advantage.

Analysis for damage:
The big issues are:
1. HotR, Avenger's Shield, Holy Shield vs. CS, DS
All protection abilities here are holy damage, and besides gaining +90% threat on use they bypass armor. CS, DS rely on weapon damage which is greatly diminished by using a one handed weapon and lose significant damage use to armor. If you take the Red Sword of Courage, your average CS damage is 252.45 before normalized AP and armor. HotR does 573.6 before normalized AP and ignores armor. DS does 229.5 damage per target before armor/AP. Note that in tank gear, the retribution paladin has no strength or AP advantage over a protection paladin (even imp BoMight, since it can be gained from other sources). Thus, before AP and Armor, HotR does more damage than both combined.

Now let's even pretend DS does similar damage to CS. Essentially, we see HotR is better than CS, so any move that is better than HotR is better than DS. Given that Avenger's Shield can hit for similar amounts as HotR and more glyphed, protection clearly wins.
*CS has +10% dmg talented, but 2pct7 is also +10% to HotR.

Given both rotations will use every GCD, there is no credibility in CS/DS > HotR for that reason. Yes, this doesn't get into fine details. Work those out if you want to dispute me. From an eyeing standpoint, ret loses.

Note: when the number of mobs are greater than 1, hotr clearly wins; with 4 mobs then every hotr,you get 3 strikes. For every two CS, you have 1 DS and 2 hotr. So protection is getting an extra hit every 6 seconds compared to ret.

2. 30% BV, 5% seal/judge DMG, 6 expertise, vs 6% crit, 25% crit strike damage on Judgment, 2 or 5% damage
I could go into depth here, but if one reads on expertise in general and ShoR "percentage of damage done" then it's fairly clear that protection is a fairly clear winner here.
Conclusion: Protection does similar, possibly more DPS while tanking in tank gear

Conclusions on damage taken
Not that there is any question, let's examine this. Given similar gear, a paladin will have say 50% avoidance and ~25% block chance. This means 27.4% of the time the ret paladin will take ~1100 more damage than a protection paladin given holy shield or redoubt are always up and 25% the time a retribution paladin will take ~254 more damage due to lower BV. This means over 100 hits the retribution paladin will take 36490 from every mob on him (that is not fully blocked by a prot paladin). If the ret paladin can full block attacks, this number is reduced to 30140. That's solely from blocking. A protection paladin also has 6% more mitigation from talents which greatly reduces damage intake. On spell casting mobs, the protection paladin takes a clear 3% and 6% less damage from talents.
With a typical four mob trash pull and 2.0 attack speed, this amount of damage can be mitigated every 100 seconds. All numbers also assume the same armor, of which the retribution paladin will surely have less of due to Improved Devotion Aura.

Note: I do not assume sanctuary on a retribution paladin since he's filling a protection paladin spot, essentially.

Overall thoughts
Can a ret tank? Sure. On trash he'll probably need more avid use of CC. He can even tank some of the easier naxx bosses - though patchwerk, sapphiron, and Kel'thuzad would certainly give him issues. Malygos 10 is not probably due to the lower HP, and certainly sartharion with adds up is out of the question since even EH tanks get gibbed on that fight - thought OT roles may work.

The content may be a lot easier than BC was and maybe going through sloppy or non-prot is doable but I take a little pride in my tankadining so no thanks to the tank-as-ret l33t crowd.

This does however shine a big light on Blizzard for their very poor judgment. I don't know why on earth they made the expansion such a cake walk. It's very disappointing. Lucky for them the game is still more fun than anything else out there.
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I'd rather DPS as Prot than tank as Ret.
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Rogues do it from behind.
Priests do it in missionary.
Paladins do it with protection.

Well I for one applaud the OP for posting their thoughts and experiences here and I'm surprised at some of the flames generated - I'm not a fan of 50g respec each way, which is why low 70s-75 I did Ret until I got SoR.

I enjoy Prot and tanking over Ret, but that certainly doesn't mean I can't use saving some time, effort and money... focusing on the time, I too PUG a fair bit and don't have a lot of it. I decided to forgo instances entirely until 75 - not that I wanted too, but it wasn't effective.

I appreciate the one-spec thoughts - we don't have dual spec in place yet which IMO would have been the MOST important thing for Blizzard to get done for the expansion, we 'd definately have less of 'Need HEALER!' spam for starters.

If there are builds/equip I can do (I too, have been saving off-spec pieces) that are a one-stop shop and still effective enough the vast majority of the time to see content I otherwise skip/don't see, why the heck not. This isn't for the guild-MT, this thread is more for the casual, even not 80 yet, like myself.

Even with dual spec we'll see how much changes - I don't have my epic flying skill yet, so if it's more convenient yet still full cost, I can't be sure I'll do it with a cost both ways there, either. I think I'm going to be pretty happy running up with my SoR, but it's also true a lot of my Prot DPS dips, particularly sustainabilty, if I'm not running as the tank.

As a final side-note my biggest disappointment not being in Beta was not being able to try the Sheathadin, so hearing Ret might cut the mustard enough slapping on the Holy set does open some thoughts - as Prot I've definately found I don't have the mana sustainability to heal much at all.
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Whack on SoW and melee the boss and your mana will regen reasonably fast! If there are three mobs to hit with HotR it will regen REALLY fast.

I'm gonna try this friday after my weekly raid. I should have sufficient gear to do it, comparing mine with the OP's.

I raid on Thursdays only and have been playing pretty much only the paladin since the expansion. He's my PVP, DPS, Tank, and healing toon atm, and I've been respeccing quite a bit, way more than I did in BC. Most of what I run is heroics, with the exception of the Thursday raid and 10-man sarth/arch when I can squeeze it in on my schedule.

If I can tank the heroics and OT the 10-mans as ret, that cuts it down to just one respec/week, and the rest of the time I can pvp/daily much more efficiently.

Before you dog me for complaining about the respec costs, there are many days where all I have time to do is the Hodir dailys and my JC daily, and occasionally crush some ore. So I can't make as much money as people with more time. If I have time to make money, I'm probably spending that time running heroics instead. 100g to go back and forth will cut a dent in your resources if you do that every couple of days.
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[url http://armory.worldofwarcraft.com/character-sheet.xml?r=Uther&n=Brute /url]