I want to put together a comprehensive list of all the Features and Quality of Life changes that WoW has seen over the last 13 years, that technically a pure Classic (Vanilla) server would not have.
Personally I am on the side of keeping Classic as authentic a possible, but there are a few things I was reminded of that I took for granted!
No Barborshop
No Name / Race / Faction Change
No Mini-map Tracking
No Deathknights / Monks / DemonHunters
No Draeni / Bloodelfs / Worgen / Goblins / Pandas
No LFD
No LFR
No raid-wide Paladin buffs
No raid-wide Warlock stuff
No contemporary addons
No Instance Maps (good luck running BRD/BRS)
No Realm linked AH (Faction based)
No Instance Summoning Stones
No AoE Looting
No Instance Re-spawn
No Guild Banks
No Regent Bank
No Flying Mounts
No Pet / Toy / Mount collections
No Transmogrification
No Void Storage
No Mage Tables
No Feasts
No Bloodlust / Heroisum
1 hour Hearthstones
Buff Regents
15 min Raid Buffs
Mana Per 5 Seconds Regen
Raid Instance Locks
Meeting Stone Queuing
Weapon Skill
Smaller Item Stacks (20)
I will continue to add to the list as people post more.