First of all, I believe that the vast majority of people one meets are normally intelligent. It is only the rare person who stands out as being not so smart or intelligent. Some of these are mentally retarded. But a few are simply not so intelligent as the vast majority of us. IQ testing aside, here is how I would describe the rare person who is not mentally retarded, but lacks what most would consider normal intelligence.
The person exhibits little curiosity. She makes the same mistakes over and over. She has little or no insight...never realizing the causes and effects in life. She wonders what happened when things go wrong, when it is obvious to her friends that she caused the problem herself. She has a poorly developed sense of humor. She never seems to get jokes...especially ones that require an awareness of double meanings, and language or slang nuances. She is unaware of current events. She often seems unaware of the significance of events around her, and may act inappropriately happy or sad. She does not pick up on subtle clues such as sarchastic responses, tone of voice or body language.
In short, the subintelligent person is incapable of normal and usual responses to conversational and social interaction.