I think there’s wide agreement that one of the key indicia that an invention would not have been obvious is that it defied conventional wisdom in the art. Rossman has this to say on the point …

"We must remember that a man inexperienced in a given field often has a distinct advantage over the men who are experienced in that field. First of all, he has nothing to lose, for his professional reputation is not at stake. He also tackles his problem without any preconceived notions or theories. He is, therefore, free to formulate his own theories or possible solutions of the problem. He is not bound by any precedent in that field and he respects no authorities, because he is ignorant of the traditions and the achievements of this field. He is less likely to follow the old groove than the man experienced in the field, for he has a fresh and unhampered outlook on the problem. Another important advantage lies in the fact that he brings to the field a knowledge and outlook that the others in that field do not have. This enables him to form novel and unusual combinations which would be considered folly by the experienced men even if merely suggested as a possibility. The ignorance of the failure of others is also in his favor for he is not hesitant and doubtful. He attacks his problem with confidence, courage, and great energy. For these reasons an electrical engineer never connected the idea of sound transmission with an electromagnet. The electrical engineer was immersed too much in his own field to see beyond it or combine it with facts from other fields. The southern planters cleaned their cotton for many years by the manual labor of slaves. They took it for granted that this was the only way it could be done, until Whitney came from New England where machines were replacing human labor and he showed them how it could be done. The carriage makers could not possibly visualize any other motive power except horses for their carriages. In the same way, we often find the experienced men in their respective fields to be conservative and with limited vision. They accept what they find and seldom question authority. For these reasons the amateur or the so-called outsider very often makes important inventions and discoveries."

Joseph Rossman, The Psychology of the Inventor: A Study of the Patentee 144-45 (2d ed. 1931).