"her heart revelled in angry pleasure, in pleased contempt, that the man who at twenty-three had seemed to understand somewhat of the value of an Anne Elliot, should, eight years afterwards, be charmed by a Louisa Musgrove." (Chapter 13)

I have always read this passage to mean that Lady Russell is being contemptuous of Captain Wentworth’s taste. :) She is pleased that he is not as discerning as he once was. Once he valued Anne, in her opinion the most worthy young lady of the neighborhood. Now his standards are so much lower, he has allowed himself to be charmed by the inferior (to Anne at least) Louisa Musgrove. I do not think she has anything against Louisa nor mind that Captain Wentworth might marry her, I believe that it is all to reassure herself that she was right in separating Anne and the Captain because if he can be charmed by Louisa he is even more unworthy of Anne.

I think it is telling that it is “angry pleasure” in which she revels. Does Lady Russell resent Captain Wentworth to this day? She got rid of him once. Is she upset that he has returned? Is she angry that she had to jeopardize her relationship with Anne to separate them seven years ago?