For a tank it's more important to spot a skull on a mob (mostly bosses) as those are generally 3 levels higher then him, at least in raidinstances.
Why is that so important? Well, those cause crushing blows which do significant more damage then general hits, 150% to be more exact.
You get this while leveling aswell when encountering mobs which are 3 levels or higher then you.
In a raid getting hit by crushing blows can cost you a tank as it is hard for healers to heal through spikedamage like that.
The only way for a tank to counter a crushing blow is too get his Avoidance to the correct level, for a level 70 player this is 102,4%. (more info here)
Avoidance is the ammount of dodge/parry/block/miss chance you have versus a mob. (a mob has 5% base miss chance at max defense)
The formula for this is: %needed= Base 5% miss + bonus miss from defense + dodge % + parry % + block %
To give you an example i'll use my own little tankie (will take the info from the Armory even tho it hasn't be updated in a while for some reason and is slightly outdated).:
- Dodge: 18,2%
As you can see this aint enough on 73+ mobs, mobs below that won't crush you anyway (assuming you're 70) so this is just an issue when you see that dreaded skull in raidinstances. We need someway to get our avoidance above 102,4%, Warriors have a nice little skill for that called Shield Block which gives you 75% extra block chance for a 1 or more blocks (depending if you took the improved Shield Block talent or not which is to be advised if you're a raider).
73,99% + 75% = 148,99% Now thats surely enough.
To be honest checking your avoidance is not realy needed when are decked in epics, it's more a problem when you are about to enter your first raidinstance and wear mostly greens and blues. You can paste the following little macro for easier checking ingame:
/script DEFAULT_CHAT_FRAME:AddMessage("Need 102.4 combined avoidance. Currently at:",0.8,0.8,1)
Try this once with and without Shield Block up so you can notice the difference. Prot Paladins have skills to boost their Avoidance aswell (Redoubt and Holy Shield give 30% block chance each), Druids have generally more health and armor so they just need to soak crushing blows.
Don't worry if you have to much avoidance as this will only make it easier to heal through, you might reach a point where you will dodge or parry too much and get rage shortage which messes up your threat cycle. Parry however makes your next attack faster so is prefered over dodge because of this.
As far as i can tell a bossmob (marked with a skull) has a skull and is always 3 levels higher then you and he can crush, but then again i'm still a noob tank and i might be wrong. What might be usefull to know aswell is attackspeed, some bosses have such a high attackspeed (like Romelo in Karazhan) that they will use up your shieldblock charges before the cooldown is over which will still cause you receiving crushing blows. Keeping Thunderclap up on this boss will result in lowering his attackspeed thus taking the boss more time to deplete your shieldblock charges.
Quite a bit of theory comes into play when spotting a tiny little skull on your screen doesn't it? :)