The talent trees from Classic, but done in a way that’s more like the weapons from Legion. A massive web of abilities and spells that you can pick and choose how you get to a few, limited options for ult abilites for all specs.
Shadow priests that have holy and disc abilities.
Resto shaman that can weild 2-handed axes and have fire elementals.
Just…massively more options and freedom. Even if there’s a “best spec must have” let people get their on their own.
From Classic to Retail: The sense of every piece of gear, every coin I pick up, every bit of armor I can craft being important (getting this would involve a lot of other things, but that’s the one thing summed up)
From Retail to Classic: The “B” button opening all of my bags instead of just the one. Oh boy that’s driving me crazy in my beta testing