A DD in defensive mode can't attack a Large size or greater enemy that takes a 5 ft step. Considering how often characters adjust their positions, whether to avoid flankers, press a spellcaster/archer, or manuever around a battle line, not being able to take a 5ft step is a huge penalty. And since a great many high level monsters have reach and thus can rip a DD to shreds while leaving him unable to counter attack. . . Equating Defensive Stance to Rage is almost criminal.
Both the barbarian and the DD get swallowed. Monsters usually have good enough bonuses to hit even armored characters often enough, and barbarians probably will get eaten before they have a chance to run away, because they charged at the begining of combat. Your example is very flawed. 15th or 16th level characters don't actually have the cash to buy +5 rings and amulets. +5 deflection and natural armor costs 100k, or 1/2 of a 15th level character's GP. If you get those, all your other gear will suck - so maybe the dragon will miss of the time, but so will you and too bad for your companions. And the dragon's +35 attack bonus is its normal melee bonus. Add in buffs (Bull's STR, maybe Divine Favor, etc), feats, and magic items for the dragon and its attack bonus will be signifcantly higher.
Defensive Awareness isn't that great. Because the DD only gets 10 levels of it, and needs 7 other levels, most enemy rogues he faces will probably be able to flank him anyway.
The +2 to Ref and Fort saves that the DD gets will usually be meaningless. How many characters aren't going to have Cloaks of Resistance at high levels? The darn things are almost required gear. High level characters will have a hard time with their best saves without them.
I actually don't like organization based prestige classes. In many cases, I find it hard to see most members of a group having levels in the same PrC. And the requirements and abilities of a PrC will exclude many people that would otherwise be fitting members. 10 level PrCs especially suck for organizations, IMHO. Look at all the PrCs in Sword and Fist that are supposed to represent training from special groups. People here complained quite a bit about the mismatching between the prereqs and the members mentioned in the flavor text section. As long as PrCs are representing an organization, there will always mismatching.
Ultimately, I treat all classes as ability templates. The unskilled but powerful warrior from a primitive tribe, dwarven battleragers, and a tough, aggressive street punk could all have some barbarian levels. The pre-supplied flavor text for classes is most useful for brainstorming when I don't have any good ideas myself.
EDIT:
Oh yeah, a barbarian can use a Mithral Breatplate +5 (10), an Animated shield +5 (7), a +5 natural armor amultet, a +5 deflection ring, Dodge, Haste and a 20 dex (magically boosted, of course) to get a 47 AC, 45 when raging. To be fair, I'll make deflection to a +4, from Holy Aura, let's say, to make up for the additional cost of the Animated Shield. So we have a final AC of 46/44. So the Barbarian is usually only 6 or 7 points behind (because Rage is almost always appropriate, and Defensive Stance isn't). While the difference can be very significant, some foes will still always miss except on a 20, and some enemies are mean enough that the difference only means a few points less Power Attack. Also, I could bring in a few other items that would help. Skin of the Hero is a really good item.
