I want to talk about how the restrictions and limitations of Classic created a far more cohesive and immersive world than what is currently on offer in retail. First off, Classes. Classic is far, far more restrictive in terms of what race can play as what class, with the only notable exception being Gnome Warrior. You can really tell a lot of thought was put into the selection of available classes for any given race. Speaking of Gnomes again, gameplay wise what they are really lacking is a healer class of any sort. Gameplay wise having Priest or Paladin Gnomes is sensible enough, but the Vanilla Devs at Blizz held off. Why? Because culturally Gnomes are very scientific. Why would they pray for divine blessings when their entire culture is based on scientific endeavor?
Compare that to Goblin Shamans. Even now, after all these years I still shake my head at this decision. Shamans are all about natural order, ancestral spiritualism, and tribal ritual. Goblins are greedy, destructive, mechanically-minded money grubbers. Ferengi in Fantasy form. On top of that Horde already had 3/4 of their original races have access to the Shaman class. There was no need for Goblin shamans. There is no believable excuse for Goblin shamans. It’s just careless and immersion breaking.
Let’s talk more broadly about narrative and world building. In Classic you did not have voiced characters and grand cinematics telling you how dire a threat demons were. Demons were a threat because you almost always encountered them in difficult areas, and these encounters were few and far between. More than that, there were Greater Demons. Elite Monsters of incredible power and demonic ability. In the Blasted Lands, or parts of Northern Kalimdor these guys were rare, mean, and required teamwork and effort to bring down.
From this and other world building you get the impression that the Burning Legion were an other-worldly demonic threat. They created the Old Horde, Scourge, and leveled Dalaran with a wave of the hand. The idea of players, even named characters, facing Sargeras was laughable. Like expecting the Fellowship to face Morgoth when they could barely claw victory from his lackey, Sauron.
But in Retail we get Legion, where the titular enemy gets beaten and bludgeoned like any other mook force, with a grand ending cinematic where we zip past Sargeras in the Millennium Falcon…exactly the kind immersion wrecking comic-book styled tripe Vanilla so expertly avoided by treating demons as the ultimate enemy, with only lesser demon spawn capable of being slain by a lone character.
There are many, many more examples of this but I think there are definite lessons to be learned from not having access to almost all combinations of class and race, as well as the merits of a far more subdued and subtle approach to world building.