Most of Stormwind's buildings that you didn't see from the air on the flight paths and couldn't reach on foot were essentially one-sided, two-dimensional set pieces with really good shading work. The same held true for every other city--Silvermoon has a few hanging out. There were also huge gaps of blocky, untextured terrain in the old Azeroth between mountain ranges separating zones and in other unaccessible areas were dev playgrounds where they would work on things they didn't want players seeing (such as the old Zul'Aman testing ground in Eversong, behind that big wall, the Old Outland playground in Deadmines, the interior of the pre-BC Karazhan, the Karazan Crypts, the terrain used for Northrend footage in Naxx40's reveal trailer, and a host of other dev playgrounds that were cleaned up and removed or permanently walled off in the Cata revamp).
But I don't think the Draenor changes are about skimping on filler terrain, especially not since, if they're playing the 'gauge player reaction' game and originally planned on flight being implemented in 6.1, they'd have to delay 6.1 or hurriedly release it before the content they want in it is ready to fill in the terrain gaps for flying if the player response is especially poor on release.