Critics disagree about the interpretation of the metaphor in "The Lady of Shalott," but some often cited themes are: restriction or deprivation; art versus artifice; love versus infatuation; and liberation. Two other themes, which some critics think are central, are that of transgression and punishment, and the deception of reality.
The theme of restriction and deprivation is seen in that the Lady is kept from directly viewing life: She is deprived of direct experience and restricted to the shadow of experience made up of whatever is reflected in her mirror. The theme of art and artifice is represented in her art of weaving and of turning "shadows" of life in a mirror to beautiful representation in cloth. One might venture to say that her representation of life in woven cloth is more beautiful than real life, because the weaving is devoid of pain and suffering, arguments and quarrels.
Artifice, defined as clever trickery, wile, craftiness, and cunning, is the opposite of art and is seen in the revelation of Lancelot's true character at the end of the poem, through his one feelingless comment on the Lady's death. Love versus infatuation is represented by the contrast between the Lady's behavior and Lancelot's. The Lady's behavior represents love because she is willing to risk all in order to gaze upon Lancelot. Lancelot represents infatuation and is pictured as coming from an unfeeling heart that is moved by nothing other than physical beauty: Seeing the dead Lady before him, Lancelot says only "She has a lovely face."
Liberation seems to be revealed in her release from the tower, but, in fact, her release only comes at the cost of the sacrifice of her life as the curse, which requires her death, is activated as soon as she looks out the window. Transgression and punishment occur in her breaking her promise to only view life as shadows in the mirror, and her punishment is the loss of everything including her life, all of which fly out the window with her weaving ("web"). The deception of reality is revealed in the Lady's reaction to Lancelot, who is the deceptive appearance of reality, and the later coldhearted reaction Lancelot has to the true reality of the Lady, who was lying in death at his feet.
The central thematic issue in "Porphyria's Lover" is Browning's exploration of madness. The theme of madness reveals Browning's controversial suggestion that madness is more like sanity than most people think it is. Another theme of the nature of immorality is explored by a comparison of seduction to murder: She seduces him; he murders her. This leads to the implied question of whether the two can or cannot be equated under the term "immoral."
The theme of power and dominance is represented in the reversal of roles between Porphyria and the speaker in the poem. He is unable to find a way to dominate her in life because she is so powerful in her own right, metaphorically able to shut out even a raging storm, which is symbolic of her power to also shut out the speaker. In order to break her will, he breaks her neck, and then she is content and happy. In juxtaposition to theme of morality in relation to seduction and murder, this power theme raises the question of the morality of the quest for power and dominance of one person or one group of people over another.
Browning is suggesting in these thematic questions that society has gone astray on the points of the mind of humanity; passions of humanity; role of equality within humanity.