I wouldn't be surprised if it was required to be level 80 so you can use your flying mounts in Northrend. Maybe like an 8k cost for the next level of riding skill?
I don't think we will be seeing any Netherdrakes outside of Outland. The dragons shown in the trailer are most likely normal dragons. I'm sure there will be quests about them espically in the Dragonblight zone, the place where all dragons go to die. Hopefully we can interact with Malygos in some way and no I don't mean kill him.
It is kind of lame that this xpack is newcomer and alt unfriendly. I still think that a great sea expansion would have new races and new zones to level up in. And it will probably have water mounts that can swim on the surface and under the sea ;D