While Paladins are not getting a spec renamed or having having a role totally re-thought like Survival or Discipline, all three specs are undergoing some serious changes that are going mean a lot to anyone who has spent much time playing a Paladin in the past. Given that they are (last time I checked) one of the most popular classes, that's probably a lot of people. And of course because most people prefer to DPS, that means Retribution's changes are going to be felt by a lot of people.
Holy Power - Retribution Remains:
HoPo was introduced in Cataclysm to give some structure to Paladin rotations after the "literally hit any ability that comes up" style in Wrath and before. While Holy (for whom it never really seemed necessary) and Protection (which is getting other rotational focuses. Foci?) are dropping Holy Power as a mechanic, Retribution is keeping it around. But Ret is not going to be just about building and spending.
Closing the Distance:
Retribution has historically had a lot of ranged spells as part of their rotation. Judgment, Exorcism, Hammer of Wrath - these all allowed you to do a fair amount of damage even if you were staying away from the target. They are reducing this by quite a bit. Hammer of Wrath is gone along with a lot of other Execute-like abilities, as they seem to want to make Execute-like abilities more of a specifically Warrior thing. Specifically the ability Execute. So Hammer of Wrath has gone poof, and consequently Avenging Wrath is a flat damage cooldown (though there's a talented replacement that is a little more complex.) Exorcism has also gotten a replacement in the rotation.
I Am The One Who Buffs:
Raidwide buffs are no longer really a thing in Legion. For the most part, players will just show up ready to wade into battle. However, harkening back to the old days, Retribution now gets three Greater Blessings. There are three of these buffs, Greater Blessing of Might, Wisdom, and Kings. You can distribute three of the buffs to any party or raid members (including yourself) in any combination, though the same buff won't stack twice on the same person.
These buffs persist through death, but cannot be re-applied or placed on another target while in combat.
Might grants extra holy damage on spells and attacks. Kings grants a periodically-refreshing damage absorption shield. Wisdom causes the target to regenerate health and mana gradually over time. While soloing, you'll want to just put all three on yourself.
Blade of Justice:
The replacement for Exorcism is called Blade of Justice, which has a moderate cooldown and hits harder than Crusader Strike. It's actually a kind of weird animation where a blade erupts from the ground underneath the target. This actually has a short range, so you can use it even if you're not in melee range, and it generates two Holy Power instead of one, which actually winds up making up for...
The New Role of Judgment:
Judgment no longer generates Holy Power. However, it is now far more central to the rotation. It now applies a debuff to the target that increases the damage dealt by your Holy Power spenders, which are Templar's Verdict and Divine Storm, plus a couple that come from talents. Judgment has a new animation (it's what Maraad uses in the Through the Dark Portal cutscene from the very beginning of Warlords' Tanaan Jungle intro at the beginning of the expansion.) It also inherently jumps to two other targets, which of course helps with cleaving.
The redesign here means that playing well is going to be about maximizing "finishers" while within the Judgment window and knowing how to conserve Holy Power when you're outside of it. There's a bit more thought you need to put into your actions, but it's relatively easy to get used to it.
A Bit Less AoE:
There's no longer a proc for free Divine Storms and you no longer have Hammer of the Righteous (that's for prot and even works differently for that spec.) You do have a natural cleave on Judgment, which will pair with Divine Storm in true AoE situations. Notably, as a talent choice, you can get Consecration back, which will of course work great for big AoE packs.
Pretty much all of Ret's combat animations have changed. There's much more of a visceral, physical feel to the spec. Even the magic hammers from Divine Storm look more solid now.
The Ashbringer itself is going to add a new ability that I believe does AoE damage, so I wouldn't worry too much if you like that niche.