The Tank's jaw at first glance seems to be missing, but upon closer inspection it is shown to be enveloped within his thick neck muscles.
Instead of focusing on one Survivor until they are dead, the Tank will attack whoever is closest and will not kill an incapacitated Survivor if there are healthy ones nearby, which makes it extremely difficult on Expert, as a Tank can easily wipe out an entire team in seconds.
When this happens, a healthier Survivor can use their faster speed (if not, adrenaline) to distract the Tank while the others rescue the incapacitated Survivors.
The open sores present before are now much larger and appear to be festering.
It should also be noted that while in Left 4 Dead, the Tank's lower jaw can barely be seen, in Left 4 Dead 2, the Tank's lower jaw is now completely absorbed by his muscle and only the tongue flops out.
The increased muscle mass serves as both the Tank's weapon and armor.