Why can't we use them in Northrend? Well, the reasoning seems understandable. They want the progression through the Northrend content to feel natural, they want you to experience the setting they've put there. You have to admit, Outland became far less dangerous to you the moment you gained the ability to fly over every single threat there is in the world.
The most common theory on the forums is that you'll need to purchase wing defrosters or wing mittens at level 80 to regain your ability to use a flying mount. A silly, cynical concept, and I don't think that's their only option. Yesterday we discussed weather, so perhaps a blizzard in the Howling Fjord could force you to the ground, or a thunderstorm in the Borean Tundra. Alternatively, a swarm of gargoyles(and frost wyrms?) could be conveniently placed to throw you off of your mount and to your doom should you fly too high in these starting zones.
There are many different things that could be done, and I'm very interested to see which route Blizzard will go with it. You know, as long as I don't need to put mittens on my Nether Ray. Where would I even put them!?