On page 447 of Norman Thomas: The Last Idealist, which is a biography of six-time Socialist candidate for President Norman Thomas, W.A. Swanberg quotes New Republic editor Bruce Bliven’s assessment in 1963 of Norman Thomas’ public speaking abilities:
“He has spent his life trying with fabulous energy trying to persuade the world to be good by talking to it. He is not a very skillful orator: his voice is loud and a little harsh; he speaks rapidly and extemporaneously; his thoughts are sometimes too complicated to be fully grasped at such speed. Yet he is enormously popular with audiences, partly because he has a good sense of humor and does not take himself too seriously, partly because of his obvious, passionate sincerity, but chiefly, I think, because you get the sense that he is holding nothing back for political reasons, that he is doing his best to tell the truth as he sees it.”
These are magnetic characteristics: not giving the audience a lot of bull, not taking oneself too seriously, being a person of conviction, etc. I think that one can be a serious speaker and be effective, however. I think of Jonathan Edwards. Edwards, like Thomas, spoke with conviction the truth as he saw it. But, unlike Thomas, Edwards did not manifest a sense of humor in his sermons, as far as I know.