There was a thing on the behind-the-scenes DVD from the Cata CE saying that, to enable old world flying, they had to basically re-do the entire map, because it wasn't originally designed with flying in mind.
They had an example of the Stormwind cathedral, that it was basically just floating in midair and you didn't actually see the Cathedral district until you were actually in the area.
They admitted to being "lazy" in the sense of "If the players are never going to see it, why bother creating it?"