I just ran through 2 "solitaire" playtest encounters to test some fixes for the Barbarian from this post. The party were level 2 (Sorcerer, Bard, and Barbarian). The Barbarian used my base HP 9 house rule, and had Slayer, Whirlwind, and Unstoppable. Feats were Rage, Unstoppable, and Strong Recovery (he was human).
The first encounter was 4 goblin mooks, 2 hobgoblins, and a bugbear. The second were 4 Lizardfolk and a homebrew 3rd level Lizardfolk Shaman (who basically has an at-will entangle).
The increased base HP probably hits the sweet spot in terms of durability.
Unstoppable is pretty phenomenal, as despite his low AC the Barbarian ended up using the fewest recoveries of the group for the two fights. Using it while Raging and/or when the ED is relatively high minimizes the risk of "calling" the shot and then missing. Still, I like the tension that potentially missing and wasting it provides, as well as the strategic considerations (when to use it).
Slayer was sort of tough to utilize (which was my player's experience in our first game as well). However, in the goblin fight I did manage to crit with it (thanks to Rage) for a hefty 44 damage. That's probably enough to finish off most staggered enemies a level 2 party is likely to encounter.
Whirlwind might be too powerful, even if you're likely to take a beating after using it. Especially against mooks, since they tend to surround PCs and you get a separate attack against each, each one dealing damage to the same HP pool. Normally I try to be pretty generous with allowing players who deal a ton of damage to the mook group to kill unengaged mooks, but with Whirlwind the Barbarian easily took out all 4 mooks (despite being engaged with just 2), and in fact dealt enough damage to take out 7 if there were that many present. He wasn't even Raging. Spilling damage onto non-adjacent mooks is likely to be a LOT more common if you have a Barbarian with this talent in your party, and it's making me think I have to reconsider where to draw that line. Upon first reading the mook rules I figured "whatever can be realistically justified with the narrative," but in practice it's too easy to do that. Perhaps I set a bad precedent by describing the "extra" mooks killed as the Fighter in my group pulling out his dagger and throwing it at one after cleaving through the others. I'll need to recalibrate, otherwise mooks will be pretty pointless vs a Barbarian (i.e. the same problem that 4E minions had when there were big AoEs in the party).
In the first fight I intentionally didn't Rage until the ED was 4 (the second I started the battle Raging). This was to simulate the fact that in a 4 encounter "day," even with the house-rule recharge rules Rage is likely to only come into play in 2 fights. It was cool to see a Raging Barbarian really tear into things, as the game in which one of my players was a Barbarian seemed cursed with unlucky d20 rolls. Like, he'd miss while Raging for 3 turns in a row. I'd never seen such rotten luck, actually.
Overall I feel pretty good about the Barbarian after the base HP 9 and recharge Rage after every battle houserules. Rage is definitely a game-changer (as it should be), and even if it's not quite as powerful as the damage-centric Fighter in my game it seems like it's probably on more even footing with the Paladin and Ranger.
An even-leveled Sorcerer is definitely handicapped until s/he earns an incremental advance, but it wasn't as dire a situation as I'd feared. Their Daily and Recharge powers are still extremely formidable, and hitting with them after Gathering Power makes you completely forget about your low at-will damage. But even at-will damage isn't so bad if you concentrate on spreading damage via Burning Hands. The upgrade from 1d6 to 1d8 at 3rd level isn't a big enough change to get too worked up about being out-damaged, and if you hit two targets you're actually about on-par in terms of damage dealt to Team Monster. While focus fire is generally more tactically-sound, it's definitely worth mentioning that AoE is the way to go vs mooks.
The Bard was cursed with perpetual bad luck. He's a Halfling so when one of the Lizardfolk hit him he used Evasive, only to reward the Lizardfolk with an 18 (which gives him a free bite). Ok, good thing he jacked the Shield spell from the Wizard list! Or not, since the third re-roll was a 19 (at least it wasn't a 20!). On top of that he hit with exactly ONE attack over the course of 2 battles. And it didn't even trigger any flexible attacks. The worst was when Befuddle was used and it missed, and it was an odd miss so didn't even trigger the miss effect. I'm definitely starting to re-consider whether "Pull it Together!" is reliable enough for the Bard to function as the party's only healer (assuming the players expect a healer). The ONE hit scored by this guy was a natural 9 (thanks to the ED), so he quite literally didn't roll an 11+ at all. Actually, he did manage it once, but it was for a disengage check. So aside from soaking up damage, the Bard didn't really do anything. It's tough for me to judge the class, obviously, but it's a class I want to like.
EXTRA BONUS COMMENTARY: BATTLE OF THE CLEAVES
Ah, Cleave. So cool that two of the core classes have a version of it (Fighter and Barbarian). It's worth mentioning the differences between the two though, because their feat support is very different. The Fighter in my group makes excellent use of Cleave every battle, and no wonder with that Heroic feat! Letting the Fighter move to a nearby enemy before "cleaving" is how he uses the power the majority of the time. In my first game, the Barbarian never got to use Barbaric Cleave over the course of 2 sessions (at which point he trained it out). It wasn't even that he wasn't getting ganged up on; it's just that he didn't happen to kill anything when that was the case. Which is a pity, since the feat is extremely impressive in that it turns Cleave into a potent offensive AND defensive option.
Granted, there are ways around this, and ways that 13th Age embraces. Player creativity is highly encouraged and mechanically rewarded, assuming you have a GM that "gets" the game. Point being, an on-the-fly "stunt" system is critical for getting mileage out of Cleave. Allies can shove more enemies around the Barbarian, the Barbarian can taunt enemies into surrounding him, etc. A lot of players might get discouraged by the lack of forced movement options (especially if coming from 4E), but they're not gone completely, they're just not spelled out. Trying to trick the enemies into springing the Cleave-trap could become a fun mini-game in and of itself.
And at Champion tier it's a lot easier, because after that Barbarians can also move before Cleaving. In fact, Barbaric Cleave is probably a better pick than either Champion-tier talents (if you don't already have it). Likewise Barbaric Cleave's Epic feat is much better than the Fighter's version. Though less reliable in the early levels, it quickly becomes better with feat support.
Truth be told, I debated between Cleave and Whirlwind for my playtest Barbarian for a while. Whirlwind is certainly more potent being an at-will method for generating multiple attacks, but the defensive penalty really shoves an already-squishy class pretty far over the edge. You become easier to hit than a Wizard or Sorcerer when you swing wildly enough to hit everyone around you. In contrast, Barbaric Cleave (with feat support) actually helps you defensively, but it's only 1 extra attack per battle (and one that's hard to trigger, at that!). Ultimately I liked that Whirlwind was more active and so I went with that. If your party doesn't have a healer, though, Barbaric Cleave is worth looking at.