A lot of quests require the use of vehicles/mounts.
For example, a lot of quests from the Argent Tournament require the use of a mount (used as a vehicle (for example http://www.wowhead.com/spell=62786)); two of the quests from the DK quest line require the use of a vehicle. The first one, http://www.wowhead.com/quest=12701, uses a cannon; and the second, http://www.wowhead.com/quest=12779/an-end-to-all-things, uses a frost wyrm (used as a vehicle)
These are just examples, as I has discovered that EVERY vehicle that works this way have the same issue:
The vehicles are completely broken on this server. When you try entering one, you are actually spawned inside the vehicle, but you don't have access to the vehicle's action bar. You can't get out either, you can't move, you're just stuck in it until you disconnect and connect back into the game.
Voilà :) Hope you can take a look at this!