This change definitely comes as welcome news to Feral Druids, who rely on high armor for tanking and are unable to reach uncrushability under current talent and gear choices. Paladins and Warriors, who use gear and Holy Shield or Shield Block to reach uncrushability, are probably less enthused. Death Knights are still working on finding their tanking groove, but since they can't use shields, it's likely the removal of crushing blows will only help them.
Those who are a bit concerned about the change may want to take heart (or find dread) in Ghostcrawler's words in another thread: Raiding is changing. In Wrath of the Lich King, they are hoping to have all 4 tank classes be able to step and tank straight up into the 25-man level (which means, in theory, that Paladins and Warriors should be getting stuff to compensate for losing uncrushability, and Death Knights should eventually have the gear and innate mitigation and avoidance to tank).
They're looking to reduce the randomness of raid encounters as well. This is their philosophy behind the removal of crushing blows as a boss mechanic. In addition, they want to make tanks do more DPS, make threat easier to manage, and make buffs to affect raids more but stack less. All of these additions, of course, should greatly change the face of raiding in Wrath.
Of course, all of this change is a bit much to process, and I'm sure that regardless of what Ghostcrawler says, a lot of tank classes are still a bit worried about their place in both groups and raids in the upcoming expansion. That's understandable, but luckily, this is a Beta. Hopefully, Blizzard keeps these new philosophies in mind as they continue on, and by the time Wrath goes live, we have 4 solid and viable tank classes, and a more streamlined, less random raid experience that still manages to be fun, challenging, and rewarding.