Mrs. Barrymore intrigues Watson because of the apparent contradictions in her character. She doesn't look like a deeply emotional person. In fact, Watson describes her as "a heavy, solid person, very limited, intensely respectable" (8.22). And yet, in spite of her apparently calm, respectable exterior, she's obviously deeply emotionally upset on the inside. Watson overhears her weeping in the middle of the night when he and Sir Henry first arrive at Baskerville Hall, and he also sees her with "traces of tears upon her face" (8.22) several times afterwards.
Of course, as we find out later, Mrs. Barrymore is crying secretly over the fate of her beloved little brother, Selden. She admits that he's done terrible things that shamed her family (like, you know, brutally murdering people). But no matter what he's done, to Mrs. Barrymore Selden will always be "the little curly-headed boy that [she] had nursed and played with, as an elder sister would" (9.75).
Watson's hasty judgment of Mrs. Barrymore as an unemotional, boring person turns out to be one of the few real errors of observation that he commits in The Hound of the Baskervilles. Unlike Laura Lyons, in whose face he immediately sees "some coarseness of expression, some hardness, perhaps, of eye" (11.4), which hints at her moral flaws, Watson looks at Mrs. Barrymore and sees a bland person married to a tyrant.
In fact, Mrs. Barrymore carries the secret of being related to one of the most violent criminals in England. And far from being a bully, her husband is so concerned for her that he risks his job and his respectability to help shelter Selden from the law. Watson's misunderstanding helps to keep up the suspense of the novel. After all, if he was the kind of character reader that Holmes is, there would not be as many plot twists to keep us reading The Hound of the Baskervilles. It would all be over by Chapter 7.