For the Record: THERE IS NO DEFENSE-CAP
A cap means that if you take more of that stat, it won't help you anymore.
There's a Block-Cap, 102.4% total miss+dodge+parry+block+5%, after which block is useless.
There's Spell-hit-cap, 17%.
There's Melee-hit-cap, 8%, for non-dual-wielders.
There's a dual-wield-melee-cap, which is very high, 32% or something??
There are 2 expertisecaps, one for dodge (around 3.25%) and one for parry (around 6.75%).
There is a resistance cap, 415 if I am not mistaken.
But there is no defense-cap, 540 defense means you are uncrittable, but defense does more than just providing that. It also gives some block/parry/dodge. And because it gives all of the three, it's one of the best stats to get to the elusive Block-Cap. Only after you reach block-cap, you should value dodge more, and after dodge, parry. Dodge scales better then Parry towards avoidance, until around 600 dodge-rating.
There's no defense-cap, only a defense-minimum.