All in all, the stat is kinda boring, as while it does still do nice things after you reach the defense cap of 690 rating (or 540 skill), most people don't bother with it and stack stamina or other avoidance. So Blizzard decided that they're going to get rid of it.
Around BlizzCon 2009, we were told that the crit reduction we formerly got from defense rating was going to be tied into things that were available to all members of each tanking class. Examples used were baking it into Bear Form for druids, Righteous Fury for paladins, Defensive Stance for warriors and Frost Presence (or rather, Blood Presence in Cataclysm) for death knights. That means that if a retribution paladin or arms warrior wanted to tank, all he'd need to do was swap to vaguely appropriate gear (or just over to a sword and shield), pop his respective abilities, and away he'd go. That's not how it appears things went down, though.