Well, I'm glad I checked back tonight!
And so, my initial thoughts, as per usual. Although I don't know how helpful I'll be because the barbarian is one of two classes I've never really had any enthusiasm for or desire to play as (the other being the monk--they're just not my thing, is all), so I have no experience with them.
Juggernaut: Huh. That's odd, since the idea has always been that the barb wanders around in hide at most. But I guess since the Juggernaut wears a giant suit of armor, it fits. I do have to wonder if it's deliberate that they can wear heavy armor but can't rage in it. I'm imagining some plate-clad warrior getting angry while in battle, calling for a 5-minute break, taking off their armor, and then going into a rage.
Furious Critical: Interesting concept! I wish fighters and other martial characters had the option to get one of these--maybe with a feat? Suggestion: I can't read, apparently, and missed the word even when I first read this. Perhaps for dummies like me, in addition to saying "At 4th level and every even berserker level after" you put a mention of Furious Critical in the class chart at the appropriate level.
However, Dazzling Prowess is kinda odd. The target isn't stunned because you conked them so hard they can't think straight; they're stunned because you're so gosh-darned cool! It calls forth an amusing visual.
Warrior Borne: Borne means to bear or carry (think "blood-borne pathogen," like that video I had to watch a dozen times at work over the years). I think you mean Warrior Born, or possibly A Born Warrior. Or maybe you are destined to give birth to, or sire, little warriors.
Forceful: You write "You can always choose to use Constitution when making Persuasion checks." You've done this a few times already in other situations, so I'm a bit confused here because you've also said that you're doing a more thorough job of divorcing skills from stats. So, could you tell me which of the following examples is more accurate to how LU is going to handle skills:
Example 1
GM: So, if you want to convince the Minister to not call for the guards after finding you ransacking his dressing room, make a Persuasion check.
PC: OK, so, using Constitution as my stat, I roll... a 7. All right, I ready my axe.
-or-
Example 2
GM: [...] make a Charisma (Persuasion) check.
PC: Nope! I'mma Berserker; I get to use Con instead! And... I roll a 7. Dammit. Well, still better than if I'd rolled Charisma.
And if it's the first example, then does it mean that I always have to use Charisma unless I have an ability or there's a rule that says otherwise, but there are lots of these exceptions?
Takes One To Know One: Well, childhood-style taunts aside (you may want to change the name), this says "When you see a creature, you know if it [...] has access to combat maneuvers." Does this mean that--DM fiat* aside--there's going to be support for monsters to know combat maneuvers? "Watch out, Arim, that otyugh knows Rapid Current Eye Slash!"
Joking aside, does this give the berserker the ability to know what sort of maneuvers an individual has ("by the way that otyugh is checking out your eyes while its flexing its claws in a way that makes it look like it's plucking something, I think it knows some way to rip your eyes out."), or just that is has maneuvers?
*I saw DM fiat because, of course, there's nothing stopping me from making a monster and giving it an ability that mimics the effects of any of these maneuvers, but that's different than saying "this monster literally knows a special martial arts technique."
(Kung Fu Owlbear?)
Exploration Knacks: Good set of knacks. I notice there's only five of them--are there more in the works, or are you actually sticking all the knacks together and therefore barbarians can get some that have been listed in other playtests?
All-in-all, I still don't have any ideas for a barbarian character, but there are some definitely interesting options here.