I think the key is that not only do we not see each other's expression and tone inflection etc to determine the 'tone' of the discussion, we also tend to only participate in discussions that we have an interest in or that are unusual. In daily communications with our friends and coworkers, we discuss everything from current affairs and politics to our pets and the minutiae of our lives. We learn overtime a person's traits which are not visible online. For instance, my coworker had to go to the vet today to have his dog euthanized. This was very traumatic for him because he had that dog for years. His children are upset as well. I know from working with him for years that he is very dedicated and reliable. He takes the time to listen to other's viewpoints. I also know his political beliefs and how much he cares for his wife and family. I know when he is being facetious or serious. Very little of that would be visible to anyone participating in a discussion online.