The limitations of science
George is a practical man. He's also a man with few emotional attachments, being that he is an orphan. So he has a materialistic world view, and to top it off, he's a successful engineer in New York. He's not just a pragmatic person; he's a true mathematician by trade. And yet, he becomes charmed by a magical letter. His wife dies of a hex when he fails to abandon his close-minded view of science to consider that maybe the universe is more mystical than he imagines. The effect is that the novel challenges the materialistic world view by way of mysticism. If George had put science in its proper place, he might have been able to save his wife.
Empathy as a saving grace
The sad twist in the ending has tremendous meaning. By showing George's failure to provide for his wife, the novelist challenges the author to psychoanalyze George to see what, if anything, might have helped him to do the right thing. One obvious solution would have been empathy. George is scientific, so empathy for him would be to consider Cocoa's perspective as completely plausible and possible, even if it disagrees with George's personal preference for science. In failing to extend this charity to his wife, George disqualifies himself as her savior.
Family duty
The most obvious difference between George and Cocoa is that Cocoa feels a duty to return home and participate in life as part of her family and town. As an orphan, George knows nothing about that, and the main idea explored by the plot seems to be George's journey to understand Cocoa's real attachment to her life in Willow Springs. In the end, Cocoa's attachment to home was of ultimate importance, since she knew about her hex from Ruby, and George does not. For Cocoa, the issue is of the utmost severity.
