I haven't really hit upon the major changes compared to 3.0 and since, but I think (by and large) they're fairly self-evident. The rule of the day is, we can kill things very well.
We have always, in my experience, been a spec that was a diamond in the rough. If you knew what you were doing, and geared yourself properly for it, retribution was always capable of reasonable damage. In the original game I topped UBRS runs, hit top 5 in ZG runs, and regularly brought plenty of damage to any 5-man. In BC I was consistently up near the top of the guild charts when I was around, superseded a bit near the end as I was rarely if ever signed up and playing, and so my gear in terms of the guild progression had seriously fallen behind. That said, however, when I was near the top of our progression gearing, I was near the top of our DPS, each and every time.
The trick was that you needed some fairly specific circumstances to bring about the damage. Warrior was a must, feral druid nearly so. An enhancement shaman with WF totem would then, on top of that, spell the difference between top 10 in a 25 man and top 3.
We were very, very dependent on those around us for our damage; without that assistance, even excellent ret players would only hit a little above an average player on the charts. The major difference in WotLK is not what we're capable of, but that we no longer need others around to achieve that excellence. If you're in a 5-man with a shadow priest, elem. shaman and warlock, you can still pump out considerable damage, rivaling the other 3 with a little knowledge and skill.
So now that we have the damage, and no longer have to resort to precise group compositions and skills to make it happen, what do we do in groups? Aside from the obvious question of 'kill stuff', you have to consider this: we are now capable of doing enough damage to get invites based on this fact alone, but we still have other hybridy skills that can make us invaluable as we always were, when played right.
Hand of Sacrifice, Hand of Salvation, Hand of Freedom, Hand of Protection, Righteous Defense. You have them, so use them!
An HoS on the tank or a squishy at the right time may challenge your own survival, but it may also save their life and as a consequence save the party/raid. Hand of Protection has its still valuable implications of dropping melee aggro for the recipient (and making them immune to it regardless), and Hand of Salvation may drive down the aggro of your threat capped fellow DPSer enough that he/she can be freed up to do more damage again.
Remember that though you now can very much specialize your class for a specific role, they haven't taken our utility away. An AoW-insta Flash of Light can save the tank's life when the healer is loading a big heal on him. A hand of freedom may free up the rogue to get back on the boss and start killing him.
There's plenty of situations in which our utility is still evident. Be sure that you use them!
And as an aside, all Ret Paladins should have Kings from here on, PvE or PvP. It's not cheap, but it's still easily afforded on top of your vital skills. My next post will lay down 2 of the best options for talent specs these days.