What we call 'common sense' can be, as briefly outlined by IR, viewed in different ways due to varying perceptions of what common sense is and what it comprises. When dealing with some situations when moderating on Com I often wonder why some people cannot apply some common sense to the predicaments or disputes that they come across in their day to day lives. However, the perception of what common sense comprises of, as mentioned, is different from those trying to mediate a situation. That can lead to misunderstandings and accusations flying back and forth over flaming and/or personal attacks.
Those who are often accused of not having common sense are those who do not face the facts and twist reality to how they want to view certain things. They could also be viewed as being irrational and stupid.
However, common sense in itself could be said to be 'common' despite the theory of 5-7% of people not having common sense. Sure enough you have those who either see the world in an irrational sense or in a different way in regards to common sense, but what common sense is always going to be a matter of perception for all of us.