On the original cash-shop thread on US forums (now locked), Maldazzar posted these interesting ideas which I think summarizes very well the issues layered with this feature:
1. Your subscription is paying for content that you're literally incapable of obtaining without further payment.
2. XP-boosts and Charms of Good Fortune are "pay-to-win", because on some level, they provide an advantage to those with more money.
3. They is a serious, logical concern that these will lead to even more microtransactions, ranging from purchasing actual armor, to even charging to access new Dungeons, Raids, or Battlegrounds.
4. The transmog-helms were a missed opportunity to actually add more gameplay to WoW, as they could have very easily have been rare-drops or rewards for new, extravagant quests.
5. Transmogrification is "meaningful content" to some players, and the fact that Blizzard is now comfortable with charging for that content means that you can no longer count on having new, compelling Armor-designs to look forward to.
6. Like all cash-shop items, the transmog-helms will become meaningless; since there is literally zero effort put into obtaining them, they hold no sentimentality, and are destined to run the same gambit as the Heart of the Aspects, which leads into...
7. Many players frown upon Pets and Mounts being sold, because so many of them would have felt more satisfying as rewards for other feats. Sparkle-pony should have been a rare-drop from Algalon, which would have completely changed peoples' perception of it, Heart of the Aspects should have been a Fangs of the Father reward, and even the new Bat-mount, players have been asking for a bat-mount ever since Burning Crusade.
8. Every other MMO to attempt a cash-shop while also having a mandatory subscription has failed, which leads some to worry that this could hurt Blizzard financially in the long-term.
9. Another worry is that every MMO who attempted the same business-model also went Free-to-Play, which many players deem undesirable, and even fundamentally at-odds with what World of Warcraft used to be, since F2P games never have a "level playing-field".
On a personal note, I very much agree with the 4th topic; July to September you will not see much action happening since there aren't any world events or 5.4 any time soon; why not create something unique and fun and implement these helms as a reward for such content instead of just putting them for freaking RL cash?