In Legion, Holy and Protection Paladins are going to be losing Holy Power, but Retribution will still be using it. I know it's a kind of controversial thing - Holy Power did not exist in Vanilla, BC, or Wrath, and a lot of people have been hoping for it to go away. I'm not among them, as I think that the rotation was pretty formless in Wrath - you basically hit all your abilities when they came off cooldown and that was it.
Ret's re-oriented much more toward melee strikes in Legion, but still has most of the same basic look - you hit with a bunch of attacks to generate Holy Power and then spend them on Templar's Verdict or Divine Storm, with some talents providing new or altered finishers (if you, for some strange reason, miss Inquisition, there's something similar to that in talents.) The main thing, though, is that you're going to be hitting things with your weapon more. Also, what I've seen of the new combat animations are really impressive (Judgment, which I think is your only ranged attack left, looks much more like Maraad's did in the "going through the Dark Portal" cinematic as you begin the Tanaan Jungle intro.)
As always, this is Alpha, so don't be surprised by changes.
Crusader Strike look identical to how it is live, 4.5 second cooldown, generates one Holy Power.
Judgment no longer generates Holy Power, but it now causes enemies to take 30% extra damage from your Holy Power generators for 6 seconds, and has a 12-second cooldown.
Templar's Verdict is the same, costing 3 Holy Power to deal physical damage.
Blade of Justice is a new attack that deals Holy damage and generates 2 Holy Power, on a 12-second cooldown.
Divine Storm looks pretty much the same, dealing AoE Holy Damage around you, and costs 3 Holy Power.
Hands and Blessings have been redesigned. Hand spells are now things that you cast on enemy targets, while Blessings are beneficial spells you cast on allies. There are a number of these. Notably, now that party/raid-wide buffs are gone, Blessing of Might, Blessing of Kings, and Blessing of Wisdom (which we haven't seen since Wrath, i think) are now single-target buffs, giving useful bonuses for DPS, Tanks, and Healers, respectively. You can have only a total of three of these blessings up at a time (and I think they have to be cast on other players.) So that flavor of Paladins helping support the group will be retained. There are other blessings, like Protection or Freedom, as well as offensive Hand Spells that don't do damage, but generally debuff enemies to make it beneficial to attack them.
Conviction is a passive that gives your Crusader Strike and Blade of Justice a 20% chance to generate an additional Holy Power.
Avenging Wrath is still your major cooldown, though with no Hammer of Wrath anymore, it merely boosts all damage and healing by 50% for 20 seconds on a 2-minute cooldown.
Retribution is a kind of flavorful passive, giving you 100% increased damage when a raid or party member within 40 yards dies. This could be very powerful in PvP, where players drop more frequently, but on raid fights it will probably not be a terribly significant boost.
So before talents and artifact traits, I think the rotation is something like this:
1. Templar's Verdict at 3 Holy Power - you probably don't want to bank it, as Conviction might send you over the 5-HoPo limit.
2. Judgment if you can do both a Crusader Strike and a Blade of Justice while the debuff is up (given that Judgment and Blade of Justice both have a 12-second cooldown, you can probably just always hit BoJ right after Judgment.
3. Crusader Strike
I do wonder a bit if this is complete. Right now it looks like Ret's only AoE/Cleave is Divine Storm, and while that's historically been pretty powerful, it's not much of an AoE rotation to just switch out one finisher. There are also a bunch of talents that are simply baseline Protection spells, like Consecration and Divine Steed. The former is something I'm sure a lot of Rets would want back after all these years (and it would make the AoE rotation somewhat more robust,) but I suspect we're going to be seeing some changes here.
Also, to be clear, this is all coming from the WoWHead Legion talent calculators, which means there could be missing abilities on their website. Still, as a look at the general shape of the spec, I think we've got some interesting things going on.