In BC, WotLK, and MoP, you buy flying at cap, though both BC and WotLK allowed you to train your non-capped people in flying eventually, making successive runs through the old content less tedious. Cata, of course, offered flying from the start, and was designed around it, IMO, for the better.
WoD, on the other hand, had zero flying until recently, and requires you to finish out a particular achievement which involves getting several factions up to a specific reputation level, along with (IIRC), all the loremaster stuff for each zone, and a few other things. Basically, you have to have already done most things worth doing in the world before they let you fly now.
If WoD had flying the same as previous expansions, I'd likely still be playing it.