In Westfall during Vanilla, you’re going up against the Defias under the rule of Edwin VanCleef, and you finally end the zone by delving into the Deadmines and collecting his head.
In Westfall in Retail, you’re solving a murder mystery, that ultimately leads to you revealing he had a daughter, who has not only rallied the disenfranchised peasantry displaced by the Cataclysm, but also other races including Gnolls.
Over in the Barrens in vanilla the entire Barrens zone only covered quests from 10-20, and you could go all the way down to Razorfen Kraul and Razorfen Downs without experiencing any significant changes in quest tone. It was all very much the same, adventure around, hunt animals, defeat pirates etc.
That same area in Retail is split into two separate zones, with the Northern Barrens having a stronger presence of the Burning Blade and Quillboar, while the Southern Barrens, which is for a completely different level range, has you go up against the other faction and explore the overgrowth caused by Naralex in the Wailing Caverns.
Desolace in vanilla is a massive, mostly boring zone, that is all grey dirt and bones. That same zone in Cataclysm has a heavy Naga presence and has a massive patch of greenery right smack dab in the middle of it controlled by the Cenarion Circle.
Ashenvale is mostly under the control of the Night Elves in Vanilla, with only a few Horde outposts scattered around (Splintertree Post and Zoram’gar Outpost being the main questing hubs). In Cataclysm the Horde controls most of the zone, with several Alliance outposts now either conquered or directly under siege and key NPCs that you got quests from in vanilla being dead. Also there’s a massive volcano that exists in the middle of the zone in retail that is not there in Vanilla.
You’re arguing from a position of ignorance if you’re saying that the zones aren’t changed enough to the point where you can still experience the vanilla quest chains and story on retail. You can’t. End of story.
Hence my damn point. If you want to experience the ‘full story’ in order from start to finish, then you MUST start with classic WoW and experience the vanilla versions of EK and Kalimdor. Then you can do TBC and WotLK before coming back to retail and experiencing EK and Kalimdor after the events of the Cataclysm, which changed almost everything about the zones.