My theory is game development is only half of the story to why people feel WoW's entire structure has changed. I believe that the game changing from Classic to Pandaria is not the only factor going into players feeling nostalgic and sad about the older game.
I believe it's also the players that are changing. At least for me, part of the fun of Classic was simply that knew nothing about the game. I explored, full of naivety. I didn't know what stats I needed, and getting to 300 in a crafting profession was a big deal for me. Even if I wanted to know (Which I didn't, because most people didn't), Thottbot's slow seaching system and random blogs were all I had to look through.
With sites like Wowhead, WoWWiki, and others making the game's information significantly easier to find, as well as players who are much more knowledgeable, I think the player base caused the longing feeling them selves (Or at least partially). The true hardcore players may have known everything all along, but it wasn't until recently the majority of the population knew so easily, too.
If I came into WoW as a newbie into the current expansion, I would probably have that same sense of wonder. It's the human condition to explore until we know everything, which in essence is defeating our selves. I argue lack of ignorance and easier access to meta-gaming strategies changed the culture.