You should consider how many mounts WoW have...

List of mounts - WoWWiki - Your guide to the World of Warcraft

People LOVE mounts, different ones, personalized ones..the more options the better. Having them do something might have to be optional to quests but at least riding in them would be great. Customizing them even more is something that would keep me very interested...kinda like furniture in FFXI.