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Aside from the updated models and perhaps new hairstyles, the only thing new will be the talk of sub-races within each faction. Though that won't be in for launch, and probably not even for the duration of Warlords. They were talking about things like orcs with straight backs in addition to the hunched versions. People were speculating that there might be High Elves for Humans in terms of sub-race options (the same racials as humans, but with the High Elf Model) since it's unlikely they'll be a race due to Blizzard only needing to change the eye color while working on an entirely new race for the horde (I.E. complaints of favoritism at that point; granted, though... the idea of high elves finally being placed in after 20+ years in lore is something a lot of alliance and horde players one -- horde specifically so they can get rid of some of the Blood Elves that would immediately make the switch since they don't fit with all the savage buildings and themes (and lore)).
Beyond that I doubt there will be customization, unless you're speaking strictly of addons such as adding books and knives to your character's armor that aren't a part of it, as detailed in an article made by them some odd five months ago. In addition to customizing your Garrison.