Crushing blows used to apply to mobs 3 levels above you, which included most bosses. The change was to make them apply to mobs 4 levels above you. If you are running an instance of the appropriate level, you will not be crushed.This is a pretty big change of course. It's hard to say exactly what this will do to the tanks as far as their roles are, but here are some situational concerns:
Crushing blows originally served a dual-purpose: they discouraged you from attacking mobs much higher level than you, and they made bosses more challenging. Now they only fulfill the first role. We have plenty of other knobs to make bosses more challenging that feel less random.
Currently, Druids are good tanks for fights that crush because of their absurd armor - at least until a warrior's armor starts catching up. At that point bears are essentially confined to tanking non-crushing mobs, where their armor is better than blocking mitigation. In WotLK, all mobs will no longer crush. However, armor on items appears to be scarce. The net result of this is that because of the armor multiplier any piece of extra armor will be exceptionally valuable for mitigation. The armor cap will be 49k for level 83 mobs; this will be very difficult to meet if bears must use rogue gear for tanking purposes and not get bonus armor on the majority of gear they have. But at least as it looks right now bears will be the kings of physical mitigation for all fights. This means that Druids are once again stuck in the same situation they were before: if itemization is good, druids will be great. If it is not, druids will be poor.
Paladins, on the other hand, get it kinda hard here. One of the strengths of paladins was their lack of crushing blows relative to a warrior; because of the number of charges on holy shield, it was very unlikely that a paladin would take a crush unless lag was hit. This is only going to improve with the overlapping holy shield ability, but now it becomes irrelevant. The good news is that block value will now be a primary stat for a paladin, so this may become a much better mitigation stat with holy shield in place, even rivaling armor mitigation for druids. Still, they are no longer the tanking type to go for if you need to make sure you do not get crushed. They're the only tanking class in the game that can tank Illidan once Lich King hits without taking a shear, however. That's an interesting change. What it means is that blizzard can design more shear-like mechanics where a certain attack must be avoided/blocked every X seconds, meaning that only a paladin could tank that. I suspect strongly we'll see at least one fight like this in Wrath, and possibly more than one.
I'm happy to see this removed. It was an odd mechanic that hindered druid tanks in odd situations and had the unfortunate behavior where crushing blows happened more often relative to normal blows the more your gear improved. With the changes to Holy Shield having overlapping cooldowns paladins no longer have to focus on keeping something up which warriors have taken for granted (uncrushability), which in turn will make their gearing choices much, much easier early on.