[UPDATED 14/10/10: Filler skill priority list updated to reflect hotfix. Significant changes.]
So, how was your Patch Day? Fraught with bugs and a learning curve which would put the average mountain stage of the Tour De France to shame? Good, I'd expect nothing less. If you've had the chance to play around with the new model yet, and perhaps even Raid, you'll probably think that things have gone backwards since 3.3.5. In some ways they have - Ret is broadly balanced around lvl85 and the two new DPS abilities we get from 81-85. You may even have lost DPS, a cursory glance at the WoL reports of a number of high-level guilds indicate a significant drop in DPS measured in double-digit percentage points. Level 80 is going to be an uphill battle in a way that the transition from Burning Crusade to Wrath wasn't.
On the other hand, Ghostcrawler has recently acknowledged that Ret and a number of specs are underperforming, whilst casters (esp. Mages) are over the top. In fairness to Mages, their mechanics are so Crit-dependant I'd expect them to be more reasonable at lvl85. A few hotfixes may well be incoming, probably to Seal and Judgement damage rather than fundamental underlying mechanics, so keep your eyes peeled. Caster PvE DPS will probably wipe the floor with yours for the next couple of months even when you get the hand the new mechanics, and as for PvP.... ahahahah NO!
This makes optimising your Ret Paladin experience all the more important now, and with any luck you'll be ready for the post-Cataclysm future when it arrives whilst still pulling your weight in Icecrown.
Stats - Strength, Haste, all that Jazz.
Normally I'd discuss the rotation first but one stat in particular is pretty critical to how it's constructed.
Strength - By far the most important stat in terms of what increases your DPS, even more so than Hit and Expertise. It's now your primary stat for Gemming.
Hit - Should be capped, preferably through base stats on gear and reforging. Every DPS-increasing ability you have scales with Hit, in addition to Divine Purpose and Mastery procs.
Expertise - Not quite as important as Hit (Exorcism, Holy Wrath and Judgement don't scale with this stat) it's still very useful for the same reasons as Hit. Cap via base gear, reforging and Glyph of Seal of Truth.
Haste - See Below
Crit - No longer as important as in 3.X, no proc'ing abilities are directly dependant on it. This is the best stat to trade for Hit, Expertise and Haste via reforging.
Mastery - At present, Mastery increases the proc chance of Hand of Light by 1% per 32 rating. It's only available via reforging and isn't quite good enough to warrant investing into over Haste or Crit.
This is very much a first pass at the Haste issue and represents my thoughts and ideas on the subject. If you have anything to add, don't hesitate to do so.
Haste is a strange stat in 4.0. As before it increases the speed of your auto-attacks, reduces the cast time of your spells and shaves off a few milliseconds from your spell GCD. However it will now also reduce the cooldown of your Crusader Strike, presupposing you took Sanctity of Battle - you did take it didn't you? If you invest in the Judgements of the Pure talent you get 9% pretty much baseline, and the Wrath of Air Totem will grant you 5% more (Sanctity of Battle is both Melee and Spell Haste). Your effective cooldown will be:
CS_Cooldown_Raidbase = cooldown modified by JotP and Wrath of Air = 4.5/1.09/1.05 = 3.93184
Your target for CS_Cooldown_Effective should be around 3 seconds plus the average latency you experience when activating two instance-cast abilities. I.e.
2*GCD+2*latency = CS_Cooldown_Raidbase/(1+%Haste/100)
|Latency||Optimum Haste Rating @lvl80|
With that out the way, it's time to talk about rotations. In 3.X the rotation was basically a very sedate First-Come-First-Serve priority system, with the odd change-up from Art of War or the Tier-10 2 piece set bonus. Spare GCD's left wriggle-room for utility, such as Hands, and self-healing via Sacred Shield. 4.0 isn't quite the polar opposite, but pretty darn close.
The way I view it, there are three parts to the 4.0 Paladin DPS model.
Templar's Verdict and Divine Storm (and Inquisition in 4.0.3) are your Finishers, analogous to Rogue Finishers. As a rule of thumb you should only cast them when you have 3 Holy Power or a Proc of Hand of the Light. However if either of those conditions are true, TV and DS are the only spells you should be casting.
This your primary Holy Power generating ability, Crusader Strike, which is pivotal to your DPS. If you have reached the Optimum Haste Rating you will cast it on cooldown, 3 seconds + 2*Latency. At haste values substantially lower than the optimum the math starts to get messy - you need to judge whether delaying the casting of CS in favour of a 2nd filler is higher DPS than doing nothing. Some quick calculations indicate that it should be worth waiting until CS is off cooldown. Essentially, you'll hit CS whenever it is off cooldown AND you have neither 3HP nor a Mastery proc active.
Fillers are DPS abilities which are cast between every CS that doesn't result in reaching 3 HP. Each is prioritised against the other on a damage-per-cast basis, which results in the following priority order:
1st. Hammer of Wrath
2nd. Exorcism, presupposing Art of War Proc
5th. Holy Wrath
6th. Exorcism (No Art of War)
This Rotation has been uipdated to reflect the changes brought in the Hotfix on the 14th October.
Using the above the basic Rotation is CS->Filler->CS->Filler->CS->TV/DS->CS. Divine Purpose procs for extra Holy Power allow you to hit TV earlier, and a Proc of Hand of Light allows you to hit TV immediately, followed by CS or TV depending on the subsequent amount of Holy Power.
There are certain situations where this priority may change slightly, the most obvious situation being when you're at >15yards range as a Judgement would allow you to close with the target more quickly, or when mana consumption is unsustainable. Note that Major Glyphs shave off some mana issues you may have.
Avenging Wrath and Zealotry should not be active at the same time so you can gain full benefit from the Sanctified Wrath talent, which allows Hammer of Wrath to be cast regardless of target health whilst under the effects of Wings. During Zealotry phases CS and TV should be cast back-to-back, but take care to ensure that you have plenty of mana before Zealotry comes off cooldown.
What does this all mean?
Well, for one say goodbye to those free GCDs, you should now be spending every GCD you have on a DPS ability. Proc-watching will also be a major part of your gameplay as you react to Divine Purpose granting you the 3 HP you need, or Hand of Light, or Art of War. In fact, the Cataclysm-equivalent of Wrath's CLCRet add-on will most likely be mandatory if you want to do effective levels of raid DPS without consuming a lethal dose Caffeine.
The problem with a full-GCD rotation is that latency has a huge impact on DPS, and that's being worked on by Blizzard. In the current Beta build a queueing system for abilities is being implemented, which should eliminate the latency experienced by chaining abilities which aren't on a spell-specific cooldown together. Until it's pushed to Live, mashing those keys is the best you can do.
One of the points I haven't mentioned is being aware of encounter phase transition timings so you can dump your remaining Holy Power
*As corrected by Antigen in comments, Holy Power only decays whilst out of combat.