Paladin is one of the best supports in the entire game in a head-on-head fight. Being very resistant to crowd control with Blessing of Sacrifice and Divine Shield AND being able to cleanse harmful magic effects from allies make their base kit extremely effective at fighting in CC-heavy skirmishes. Still, they only had one viable spec, Holy. Retribution was immobile and undertuned, and Protection was a confused mess. Both of these trees got a lot of love.
Retribution was somewhat of a joke in Classic. Your friend swears he oneshots people in Arathi Basin, but you've never seen it, and you've also never seen him cast Blessing of Freedom on another target. We've given a little love to their talent tree to offset some of their mana and mobility issues, giving them more opportunities to be the very simple RNG death machines they were meant to be. "Wheels of Justice" gives targets afflicted with Judgement of Justice a 100% movement cap speed. This, in tandem with a 15% passive movespeed bonus, means that Retribution Paladins have a niche in Warsong and can stick to targets a little more, somewhat offsetting their natural Achilles Heel, being kited. They also get undispellable Seals, making counterplay to a Ret Paladin a little less one-dimensional. Their flashiest talent "Improved Hand of Freedom" changes Hand of Freedom such that, when the Paladin casts Freedom on a friendly target, he also casts a lesser version of it on himself. This allows Ret Paladins to support their teammates without sacrificing their own ability to stick to targets, allowing them to be a key component in Melee Assist trains. All these changes pale in comparison to the biggest buff to Ret Paladins, which is the easier/more optimized gearing and the cross-faction nature of the server. For the first time in WoW history, Paladins can benefit from the Classic-era Windfury totem. Windfury and Seal of Command have a multiplicative synergy. More attacks means more Seal of Command procs. This alone can allow for some truly explosive, game-ending damage.
Protection in Classic was widely considered the worst spec in the entire game. It had no place in PvE or PvP. It couldn't tank, it couldn't heal, it couldn't DPS. It truly had no place and no identity. Outside of Reckoning and Blessing of Kings it had nothing defining, interesting, or powerful in any facet of the game. We gave this spec a lot of love. First, the capstone talent is "Compel", a long range taunt on a 1 minute cooldown. "Taunt" is a new form of PvP crowd control in Legionnaire+, drawing targets into melee range to attack you. This can be extremely useful in drawing an enemy rogue off of your flag carrier or healer, or pulling key targets out of an enemy mob in order to focus fire them down. Protection Paladins also get "Sacrificial Aura", redirecting 10% of the damage your allies take to yourself as physical damage. This works in tandem with the new "Improved Reckoning", which allows you to generate stacks of Reckoning by blocking incidental redirect damage. A new ranged nuke, Avenger's Shield, and a melee interrupt, Rebuke, ensures that they are not completely safe to ignore in a group fight either. This is by far the most reworked talent tree. We think this is enough to give Protection Paladins a new identity as a sturdy, interrupt-y, versatile support unit.
Holy Paladins did not change.