According to reports, players will have to venture forth by foot until they reach level 78, at which time, flying mounts will be usable. Blizzard poster Bornakk came by and answered the OP:
From a design standpoint, we don t want people to just fly over continent and skip things when they are leveling up. Instead we want people to explore and experience the environment around them when they first reach the shores of Northrend. You will walk (or ride) from one quest to another as the zones unfold before you instead of just flying over it without batting an eye.
Initial comments were that flying mounts were going to be removed, until a prolifera of critisism resulted in Blizzard having to update comments/(change plans some, perhaps?). When I first saw this discussed, I could just see it, with people having payed 5,000 g for 300 riding, then turning around and...
Only thing I see left, is the relative size of Northrend. Lets just say some of these realms are quite populated; and forcing many who just tollerate each other but at times get cranky into small quarters can have human interaction issues as they must all compete for less space. I'm actually hopeing this is foreseen, and some stepps, even to just add some filler content/space is taken into account so contention doesn't grow.
People in dailies, can grow contentious enough, such as the one horde who landed (on my Nelf hunter then), and went to try to ninja my aether ray for the "wrangle more rays" quest. When I kited it away, and it was obvious he couldn't just get in range to laso it, he took to killing my ray instead. After following me around a bit, I finally just agroed a whole bunch of bores, walked up to him, then feigned death. He left me alone after that.
When BC first came out, and everyone was creating Draenie and Nelfs, starting areas had over 1,500 players in them. Things were such that it could take over an hour waiting for respawns to get the mobs for a lvl 1 quest. People with 60 mages, were offering to give free portals to some, to get them into other newb zones (such as Ellown Forest) so we weren't all competing over the same mobs. It'd be like that, if too congested, with GMs getting the brunt in TTs filed from one player against another in increased proportion. Northrend is essentially a small continent.