Mine? Hm...
- Being able to toggle to modern graphics, models and animations. I miss Classic systems, design, etc... But hell I surely don't miss the 3 polygon spells.
- Having the barbershop. I actually forgot to realize that barbershop wasn't a thing in Vanilla, and you were locked with the hair choice you made at the start.
- The full modern interface.
None of these actually interfere with the game systems themselves.
There is another thing that in "theory" wouldn't interfere either, but I know many players that draw a line because of visual cues of power: transmog system. That would be the only selling point that would make me fully change Retail (at its current state) for Classic 100%.