Head enchants are gone.
We decided that putting head enchants on the various faction vendors wasn't working well with our design intent for the factions. There is a lot of max-level quest content in MoP, much of it focused around the reputation with the new factions. I think it's really cool stuff -- worlds away from the old Argent Dawn scourge stone days.
In previous expansions, the head enchants on the faction vendors served to force players into that content. You couldn't even choose which reputation to pursue -- you had to pursue the one with your specific head enchant. Our design intent for MoP is to give players options in how to play, and the head enchant design wasn't compatible with that.
Once we decided to no longer offer new head enchants, we made the older ones non-functional or else players would feel like they had to go back to older content or be missing out on power. This way, helmets are just no longer enchantable and you'll have one less required step to get a piece of loot ready to wear.
Shoulder enchants can now be provided by scribes, so those still exist, but they are also not part of the faction reputation system.