Bruhnic is right in many aspects, but mainly with HC characters. They spent over a year fleshing out the first 2 acts and balances then added the 3rd with very little testing aside from alpha -- which is the real reason you cannot afford to lack in the defense dept.
However it doesn't limit build options too much, but definitely makes you take a whole lot more defense than it should and is VERY gear dependent.
Also, once you come to grips with the system a bit more, you'll make specialized builds for different areas or farming or pvp which don't include the limitations you have for progression.
The link Verno linked is the one, think you just add the .hardcore. before the domain name iirc.