Here is the essential problem: monsters scale by level.
AC is Level + 14, and PC to hit is around level+7 (give or take) (+4 stat, +2 weapon proficiency, floating +1 from proficiency, feat, or what have you)
This roughly means (after everything is said and done) that an equal-level PC will hit a monster 50% of the time. For every level of difference, that moves 5%.
Damage from monsters is on average level+8, which roughly equates to a healing surge on hit (for everyone but a defender)
Missing most of the time 65%+ is boring. Especially as that means you're more likely to miss on your important encounters and dailies. Missing more often than the rest of the group is even worse, because it's essentially saying "You suck, go find a group of your own level." At the same time, being significantly over-leveled with regards to the monsters is boring: they can't hit you, and even if they do, they don't do much damage.
If you want to play with mis-leveled parties, get rid of the auto-scaling... everything (except for maybe damage), otherwise the lower-leveled members of the party won't be useful and the higher-leveled will have no challenge.
Sometimes a skill-focussed player can bypass entire obstacles with that skill. This is a shining moment for them (which you don't want to step on), but boring for the rest of the group.
The general principle I'd follow here is "Yes, but...", useful throughout GMing: Don't say no, but do say what obstacles arise as a result.
First, take a look at the rules. The skill description for Intimidate says that it can "force a bloodied target to surrender" or "cow a target into taking some other action".
But that doesn't mean you control them. It doesn't say "unconditional surrender"; a typical minion might, but a King is likely to demand parole, the right to keep his weapons, and free passage for his men to leave - things the players might not be willing to grant. Also, the requirement for "bloodied" really means "the party has proved a convincing threat". I'd allow this check against non-bloodied targets in highly intimidating circumstances, and disallow it (forcing the second use of intimidate) in some cases - "berserk opponent" is pretty much an exemplar case.
Likewise, "taking some other action" just means that he doesn't make the attack he was planning. Maybe he gets cautious and retreats until he can gather more men. Maybe he agrees to negotiate, but he'd be negotiating from strength and likely to make demands the players can't agree to. (If this happened at the start I'd consider having him fight as normal, with a penalty for being uncertain - which goes away when he berserks. But you'd passed that point.)
The question to ask when you're deciding how he's cowed into taking some other action is: How does this guy behave when intimidated? Not all people react to fear by panicking: Some scream and run. Some keep their heads and run in a controlled manner. Some try to negotiate. Some look for another plan. Some get defensive and hostile. Some people get aggressive and hostile.
(And either of these last two are pretty likely for a berserker king. You said he wasn't immune to fear; you never said he didn't have berserk-refusal-to-surrender. Maybe he berserks, but has a -4 to defences the entire fight because he's swinging so aggressively that he doesn't defend himself. And he'll definitely attack the bard first - even if he's taking Mark hits from the Defenders for it.)
Finally, there's always "say yes, but reroute the plot to your ends". Suppose he's genuinely so scared of the group that he surrenders - how do his men react to this? Barbarian kingship involves keeping your throne by force - if he seems weak, probably his chief commander is outraged by his cowardice and challenges him for the throne on the spot.
Describe a fight, and then whichever of them wins is angry enough to attack the players immediately - but they're already injured and berserk from the fight, so the players get to skip the first half of the combat, and they've used some of their powers up.
Or if he surrenders in return for letting his men go... another warchief is going to take them and attempt a rescue, or use the evil plan for himself, depending on loyalties.
That's the principle - let the party benefit from the skill, but the benefit isn't "miss all the fun".
If you want to avoid this situation in the first place, there's more work to do. A player who is entirely focussed on one skill will tend to beat any reasonable skill check you can throw at them. This isn't a bad thing; the opportunity cost of being that skill-focussed is not small and the player should get a benefit out of it.
For that reason, I would advise a solution other than "make it impossible in general". If you give every solo immunity to fear, you've not only removed the usefulness of Intimidate, you've also removed a range of possible villain personalities. Some arch-villains are cowards! (Not this one, obviously.)
(A berserker king probably should get Immunity to Fear when he's berserk anyway, so you'd be fine - but in this case you'd already passed that point. To give it to him all the time undermines what "fear" is.)
Instead, give solos resistance to fear. Make the player work for it.
(As GM I'd rule that the general +10 for "Hostile" isn't enough here. He's not just "enemy soldier" hostile, he's a berserker king being challenged at the height of his power by enemies who oppose a plan he's been putting great effort into, in a way that threatens his throne if he seems weak. That's good for a large circumstance bonus any day.)
Also, chaotic or neutral enemies may not feel bound to honour their surrender, if they get a chance to get loose.
Best Answer
The answer to your question is "yes."
This is clearly a communication and expectations issue.
Here are some things that will help your group:
A DDI subscription if you don't already have one. This gives you a searchable rules database and should help put everyone on the same page. Also the online character builder can help get everyone a character sheet with all of their powers and abilities in front of them. It also helps with the math.
Clear communication between group and DM. If you are unhappy with loot payouts talk to the DM. You might want to talk to him about why specific magic items in 4e are important (the math gets quite wonky if you don't have level weapon/armor/neck slot). If he wants to make magic items rare in his game talk to him about intrinsic bonuses to offset the imbalance (see the dark sun rules for this).
Failing to get these bonuses will make encounters harder as levels increase. The math already favors the monsters as levels increase even with magic items and full accuracy and damage feats (see this answer for the maths). 4e as designed relies no players and monsters being on basically the same footing as you level up, forgoing magic items puts the advantage fully in the monsters' court.
Suggest that your group form a social contract. It doesn't have to be written down, but it would help. You already have an implicit one, just firm it up into something more explicit that will help you to talk about expectations for the game.
If you think the DM is being unfair or unjust talk to him about it and make sure he remembers the first rule of DMing: You are there to facilitate the story, not to screw your players. Ultimately D&D is about the PCs winning (most of the time), if your DM forgets that it can get ugly.
Finally communicate, communicate, communicate! but don't be a dick about it. Nicely and politely talk to the DM about why he isn't dropping loot, ask him if there is a reason? maybe there is. Talk to him about the math, if you need the evidence look around here we do math pretty well.
Lastly remember that A. its just a game and B. its supposed to be fun. If you are having fun, why are you worrying? if you aren't having fun then talk to your DM and see if you can make it more fun. If not, it might be time to find a new group.