I don't get why folks assert that letting a barb use their Con for AC will "turn their AC from bad to incredible". Barbs previously used their third-best stat for AC bonus, now they'll use their second-best. Shouldn't be a huge leap.
When the Barbarian Playtest was first released, people noted that while Barbarians were supposed to wear hide armor, neither their primary nor their secondary stats helped keep their AC on par at higher levels. That's why the designers came up with Barbarian Agility, that was supposed to make up for the 6 bumps to dex or int they missed over the course of 30 levels ( +3 AC in light armor, +3 Reflex defense).
However, with this feat, you get the best of both worlds: when you bump Con, your AC increases, and you still gain the bonus from Barbarian Agility.
This means thatm assuming he takes Hide Expertise and focuses on Str and Con, a Barb's AC will be 1, 2 and 3 points higher than expected in heroic, paragon and epic tier respectively. In addition, Con increases his HP, his Healing Surge value and the number of his healing surges.
Add Second Skin to the mix, and his AC at higher levels is higher than that of your average 25th-30th level shield and board fighter.
So, while the feat is nothing to write home about in heroic, by the time you're into Epic Levels it's probably worth 6/7 points of AC.
Basically, if it wasn't for Barbarian Agility, the feat would probably be fine.