As many have stated before, there are a big diffrence between QoL changes and game changes. I played in Retail TBC from patch 2.3 and would love Dual spec being implemented.
Make respecs more expensive (higher roof, same ramp up), and have dual spec if we are concerned about gold dumps.
Barbershop could cost 50 Gold, if it's not implemented for points instead.
Flying mounts could easily be 100% instead of 60% as there is really just an annoyance of having 60% flying and 100% ground mount.
All things above are just annoyances. We can live without them, and we will probably play without them. But they are annoying to many, and not really nessecary to experience the game "correctly".
However, RDF will dramaticly change game experience since you get teleported to dungeons and take away alot of the community not requiring you to socialize and looking for people/groups to join. And many would probably love dual spec and hate RDF at the same time.
Dual spec, Barbershop, Training dummy, Transmog and 100% flying mounts are QoL changes not impacting the game significantly.
RDF, 500% flying mounts, teleports and what not are things impacting alot, and should not be implemented.