The first and most striking similarity between Huckleberry Finn and Daisy Miller is that neither cares a whit about social norm - what is proper; what is expected of them. The appropriate behavior of the day is neither acknowledged nor appreciated. Huck continually struggles with Miss Watson's rules for living in her house - clean, starched dress clothing, formal table manners, early to bed early to rise. He is bewildered by how a tight collar is better for a body than loose-fitting clothes one can move in, or how a fork can possibly be easier to use than one's hands. In Daisy's case the rules being broken were not quite so simple. She has no problem using table manners or dressing in appropriate fashion. Her rebellion comes in the way of drawing room manners. She wants to run around with whoever has her fancy at the moment; she sees nothing wrong with holding several suitors at once. The whole of appropriate society gossips left and right about her inappropriate traversing with so many men, and yet she can do nothing but laugh at them and be on her way. She goes so far as to throw it in their faces per se, by running around at all hours of the day and night alone with one or another of her suitors.
What I would have to say is the most socially significant similarity Daisy and Huck share ties into the previous similarity. While neither of the two cares about social norms, there is the bigger picture not of two rebellious children, but rather of two independent minded, liberal people struggling against a closed minded, conservative society. Both have fresh, new ideas - Daisy with her liberated womanhood, and Huck with his personal freedom,