In Wrath of the Lich King, one of your characters had to reach max level to acquire flight. After doing so, a BOA book was purchasable from the flight trainer to learn flight on your alts at level 77 (3 levels below Max at the time, and 9 levels after the minimum requirement for access to Northrend).
**I do NOT remember if this was instilled at launch or if a patch instigated this**
** EDIT added thanks to poster. Flight was available from a trainer on your first max level character as of level 77 - Point of post remains valid despite error**
+It provided a gold sink.
+It made players experience the content from the ground as intended by designers.
+It provided an advantage to Altaholics while not detracting anything from players who do not play alts.
+It would give Blizzard ample time to reasonably have flight available in WoD.
Why is this model seemingly being ignored? It it is a great compromise between several point-of-views regarding flight in WoW.