According to WoW europe web site released, in the new mechanics you can

expect from
the expansion is the new vehicle-based content, which puts an

interesting spin on some 
of the concepts originally developed for PvP siege gameplay. Certain

types of vehicles 
and mounts can now carry passengers, making it possible for players to,

for example, 
break a prisoner out of a tightly guarded compound by attempting a

daring escape on 
horseback. 

While the NPC steers the horse, the player lobs explosives at a pack of

pursuing worgen. 
In another quest, the roles are reversed as the player takes to the

skies in a flying machine
to track down and rescue survivors of a catastrophic event. Even beyond

the ability for 
vehicles to hold passengers, Wrath of the Lich King introduces an