Good news, everyone! Ghostcrawler has confirmed that head enchants are coming out of the game for Mists:
"Head enchants are gone.My initial reaction was positive. There would be no more angst because the faction with my head enchant was not the same as the faction with my shiny exalted gear! I had a little think though, settled down a little, and realised it wasn't all rainbows and celebratory cakes.
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 Crusade 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." (Source: Wowhead News | Blue post)
The trouble for me is that the grind to exalted for that head enchant was content, something to aim for, another thing to achieve. I live for achievements, UI-based or self-imposed. While I'm not losing much with the removal of this grind, it was still content. Couple that with account-wide achievements removing the necessity to grind mounts/titles/achievements on more than one character if you don't want to, and I find myself experiencing a twinge of worry.
I do find it harder to mourn the loss of shoulder enchants obtained via rep grinds - Therazane rep and its forced series of quests to even unlock the tabard mean that only my main has friendly status - and as they will be available from scribes I don't think the change will be felt in the same way, but that's another milestone we don't need to reach anymore. I'd always wished for an Inscription option on top of the Therazane grind, or at least Bind to Account status for the enchants. We've gone straight past that to just removing the rep element altogether.
Ultimately, there are so many new features shipping with Mists that it's been impossible to see how I would run out of content. Pet battles, farming and more have been added. All the usual things will still be in the game. I will simply have to trust that Blizzard have got their balancing right!