The final PDF of the 13th Age core book is here, and unfortunately there were no changes made to the Barbarian. I had my hopes up when they were releasing individual chapters with the completed layout and the Barbarian was the only class conspicuously absent from these "previews," but alas the Barbarian is still underpowered.
I've thought about how to fix this class a lot in the past. I wrote an article on some options for doing so in the 13th Age Homebrew blog. I've discussed the issue with others on the Pelgrane Press forums and in the Google+ community. There were a lot of thoughts on the issue, and I've come to some additional conclusions (subject to refinement, of course).
The Defense Issue
This one's the biggest problem. Barbarians have absolutely awful AC for a frontline melee fighter, only moderate HP, and will attract a lot of attention given their high damage. On top of that, a few of their best talents make them bigger targets (Slayer can draw opportunity attacks if used to maximum effect, and Whirlwind lowers their pathetic AC even more). My previous solution, from the 13th Age Homebrew article, was to introduce a Talent that generated temp. HP when Raging (Berskerk Vitality). Some commented that it made a simple class potentially too complex, and it admittedly did widen the gulf between a Raging and a non-Raging Barbarian.
One suggestion that I got was to increase the Barbarian's base HP to 9. I keep coming back to this idea when thinking about how to homebrew the class back into effectiveness, and I think it's probably the best solution. Rogues make up for their base 12 AC by having Momentum powers (crafty fighting); Barbarians should make up for it by simply shrugging off hits. While THP does accomplish this it's a little more roundabout than simply giving the guy more HP. No messing around with new talents, which don't solve the problem of making all Barbarians more survivable anyways. I'm not sure if the HP increase will be enough on its own, but I think it's a great place to start without diverting too far from the core rules by making up wholesale new mechanics.
This is the signature ability of the class, and the recharge mechanics are a little weird. You only get 1 recharge roll instead of 1 after each battle, reducing the likelihood of using it later on in the day. Even if you get to add your Con mod to the roll this is a bad deal. I still stand behind making it follow the general recharge rules and rolling after each battle (this'll also reduce the chances of players "saving" their Rage; that's not how a Barbarian should look at things!). This serves the double purpose of making the Rage feats a little less mandatory, since you'll be more likely to Rage in multiple battles per day anyways.
Another interesting mechanic can be drawn from the D&D Next playtest. Barbarians have the option of making a "Reckless Attack" when they're not Raging, which gives them the offensive punch of Rage but at a serious cost in defense. I'd LOVE to see this tacked onto the 13th Age Barbarian's Rage, though it might be a little "much" in combination with the Rage feats that already exist. This might actually be a good avenue to explore via a new Talent (again, trying to avoid messing with the core Rage rules as much as possible). Here's a draft:
When you're not Raging you can make a basic melee attack as if you were in a Rage (that is, you'll roll 2d20 taking the better result and critting if both are 11+), but you take a -4 penalty to AC and PD until the start of your next turn. This action cannot be taken during the same turn that you use Whirlwind.
Adventurer Feat: Once per battle an enemy you hit with this attack is Vulnerable to your attacks until the end of your next turn.
Champion Feat: Once per battle if you hit with this attack the target is Dazed, in addition to being Vulnerable.
Daily Feat: Once per battle if the attack hits you can heal using a Recovery. This can be a different use of Wild Attack than the one that triggers the Adventurer/Champion feats.
Other Suggestions - More Damage
Another popular house rule I saw suggested was to give Barbarians a d12 damage die with big two-handed weapons. In combination with the other boosts I think this might start to be too much. Feedback I've heard from other groups indicated that Barbarians already deal a ton of damage. The reason that I never saw this in my group was because the Barbarian player kept rolling low (missing with both d20s even while Raging surprisingly often), and because he kept getting knocked out. With more typical dice rolling, rolling Rage recharges after each battle, and the boost to base HP 9 the Barbarian should be improved enough to be competitive. If you try these out and still find the class lacking, increasing the damage die is a really easy fix that doesn't introduce any additional complexity. For those that want more variety in Talent choice, Wild Swing or Berserk Vitality might be good options to tinker with.
Summary for Houseruling the Barbarian
Start with bumping his base HP up to 9. Additionally, let him make his Rage recharge after every battle (as per the general recharge rules).
If this doesn't sufficiently improve the class to parity, boost their 2-handed damage die to a d12. You might also offer players the option of taking the Wild Swing and/or Berserk Vitality homebrewed talents.