Speaking strictly personally here from the POV of a Ret Paladin -> from classic to current:
There are elements of the spec that I miss from previous iterations, but overall the spec is considerably more engaging to play now.
- Classic Paladin -> Really don't miss any elements here, except maybe the concept of Judgment consuming a seals effect to do a bigger effect.
- BC Paladin -> I missed most of BC so no input here.
- WotLK Paladin -> I miss the proc that allowed Divine Storm to be used in ST rotations. I also missed Righteous Vengeance dot rolling. I miss Seal of Blood/Martyr. I liked the concept of hurting myself to deal more damage because I had tools to offset it as well.
- Cata Paladin -> I did not like Holy Power or Inquisition. I don't like bland maintenance buffs unless they impact gameplay in a non-numerical fashion.
- MoP Paladin -> Skipped beginning of MoP - not much insight here. The pacing of Ret was really solid though through SOO.
- WoD Paladin -> One of the best playing iterations IMO. Divine Storm and Final Verdict and Seraphim all cool and fun interactions. I sincerely miss FV all the time. Seraphim wasn't my favorite, but it had decent gameplay.
- Legion Paladin -> I quite liked Divine Storm Tornados and a 30s Wake of Ashes generator. I didn't care for the Judgement treatment arms copy/paste stuff, but Divine Hammer and Crusade were very cool. I was also a really big fan of Blade of Justice aesthetically. Some Legiondaries had really cool effects, cape, ring, bracers, etc. had neat interactions.
- BFA Paladin -> Glad to have Hammer of Wrath back (bleh talent), and the Legendary cape from Legion makes a return as a talent, albeit in a less versatile form.
So - in all I have to say WoD Ret was my personal favorite. it had a good number of buttons and procs (arguably too many procs), but the gameplay was always fun (shame the expac wasn't).
I'd probably polish up the talent tree to find a way to get these effects back:
- Legion's Divine Hammers
- WoD Final Verdict
- Legion's Divine Storm Tornados
- WoD/WotLK's Divine Storm procs in ST