I’ll start:
• Classic: Immersive world – should be doable without slowing the game down, just needs a creative modern approach.
• Wrath: Valor points – target farm gear instead of endless RNG.
• Cata: Reforging – sucks getting loot with the wrong stats.
• MoP / Legion: Class design
• WoD: Leveling
Cata’s best feature was Have Group, Will Travel.
Do want.
That was a BC feature.
People also love to forget that the gear you got with badges of valor was garbage, we still have a vendor selling garbage gear but nobody uses it because now more players know the gear is garbage
I agree, the Obsidian Fragment system in cata is pretty decent tho.
Bonus rolls was also great.
BFA- Twilight Devastation (corruption powers)
I would love to see this come back and miss my sky beam of melting. 
Well you’re in luck
Vanilla - questing style
TBC - horde get paladins and alliance get shamans
Wrath - multiple raid difficulties
Cata - transmog
MoP - Cross Realm Grouping
WoD - Flex raiding for LFR/Normal/Heroic
Legion - Mythic+
BFA - Horrific Visions
SL - ??
DF - Crafting System
TWW - Delves