Nor are untyped bonuses to defenses in those items "broken", but they still break promises we were given about the balance of items in 4e.
And they certainly widen the gap _considerably_ between a group that gives out all of the items from Adventurer's Vault and one that doesn't. Nevermind the group that plays much lower magic and doesn't get much beyond a basic enhancement bonus progression.
I am not sure yet, but it appears to me as if at least those masterwork items that grant a bonus to other defenses just force you make a choice between another bonus to AC or a bonus to one of your defenses. I don't know about other items.
What I find more useful are certain potions and elixiers. For example the one that grants the party resistance 5/10/15 against necrotic and poison damage. Very useful!
But for balance concerns I am looking more at items that seem to change the amount of modifiers/abilities that you can stack. Being resistant to all energy types is quite useful, but will it turn out game-breaking?
Getting an "untyped" bonus to Fort from your armor seems to break the balance - except when it doesn't, and it effectively means you just can't improve another defense by that point. But if you invent multiple items that do this, things might go wrong.
But then, I notice that Masterwork Armor increases AC more then Cloaks increase your other Defenses. So to fully account for the games math, you need something to improve the other defenses, too. (Unless Masterwork armor is broken, too)
Monster attacks increase with +1/level. PCs defenses increase by +3/4 per level (+1 per 2 levels automatically, +1 per 4 levels from magic.), plus potentially another +1 per 5 levels for those defenses you increase per default. This leaves us with a gap of up to 4+ points at higher levels if we don't have such items - are these expected to come from powers, or are they expected to come from magic items?