Vanilla and TBC still had pretty iconic warcraft "candy-graphics", the colorful world and items were stylished. If you compare almoust any vanilla content, its vastly different beginning from wotlk. Nowdays all items and every area in the game feels bland in terms of colors. Why did blizzard move away from the candy-graphics that made the game look more unique? Wow has become gritty dark. Also the models were better looking in vanilla, they had certain style which is not present anymore in current game. Also starting from MoP, the texturing style has shifted into "painting" style more.
In short, vanilla had hugely better feel of the game world.