My mistress' eyes are nothing like the sun.
-- Speaker (Line 1)

Importance: In this first line, Shakespeare sets up what this poem intends to do in both language and meaning. He uses a stock cliche of romantic poetry (praise for the lover's eyes) and a familiar metaphor (the brightness of the sun) in order to neatly reverse it. The strong language of "nothing like" introduces the comedic tone of the poem.

Why then, her breasts are dun
-- Speaker (Line 3)

Importance: In this line, the speaker reveals that, contrary to the expectations that beautiful women ought to be pale and fair, his mistress is darker-skinned. Specifically, her breasts, a symbol of feminine beauty, are "dun," or grey-brown, in color (3). This functions as much as a critique of the beauty standards as it does of the woman in question– after all, skin is not snow, and perhaps, the speaker suggests, it should not have to be.

No such roses...
-- Speaker (Line 6)