1. Lacking Curiosity. Most people think that those who lack curiosity are usually uninteresting since they have no real interests.
2. Asking Too Many Questions. Many people think that someone who are too concerned in someone's life or who ask a lot of questions are dull.
3. You Exclusively Discuss Work-Related Matters. It's safe to assume that you're a great person if you love to talk about your profession and are a workaholic!
4. Being Without Interests in Anything. If you don't have any interests at all, you belong to the dull people group!
5. The "My Life Is So Interesting" ones. A commenter says that if someone is so interested in themselves or all they can speak about, it may actually put you to sleep.
6. Lack of sense of humor. Speaking with someone who has no sense of humor at all isn't enjoyable.