1. People who are spontaneously funny are often unable to tell jokes well. Conversely, people who tell jokes well are generally not spontaneously funny. The reason for this is that inherently funny people do not need to rely on "jokes" to get laughs. However, in order to get laughs, the unfunny person is forced to learn them.
2. People who are smart and confident feel comfortable answering questions with a reply of, "I don't know." People who are less confident or less smart often feel the need to answer all questions, regardless of their lack of knowledge on the subject.
I may add to these in the future.