a LOT of quests have pre reqs. the game is still new enough that a lot of the quest info online doesn't list all the requirements either.
say you want a quest from ogrimar, but you are undead and the NPC doesn't seem to have anything to say to you - try blazing thru the valley of trials and razor hill quests to see if maybe one of those is a prereq. this could be a pain in the ass, but if you REALLY want the quest, it might be the thing to do.
or you can check out quest info at thottbot.com, wowvault.ign.com, wow.allakhazam.com, or any of a number of websites with conflicting info about quests
