Recently, CM Bornakk explained the reasoning behind that unpopular decision. Essentially, they don't want players flying over the early content. The entire expansion is designed with a certain progression in mind from zone to zone. Flying would make that irrelevant.
There will be a point in the expansion where flight will be enabled in Northrend. Blizzard is currently considering level 78, but hasn't made a concrete decision.
What do you guys think? Should flight be enabled immediately or should you enjoy the early zones flightless?