If snow be white, why then her breasts are dun; (line 3)
This line points out a particularly unrealistic stereotype about women's beauty. Skin that is literally as white a snow would probably be scary. The speaker lets us know that his lover doesn't come close to that spooky ideal. He also goes out of his way to refer to her breasts. He could as easily have said, "her skin is dun." This choice emphasizes the importance of her femininity, and encourages us to think about how poetry depicts women. While we're on the subject of appearance, we should point out that there is a definite racial side to this talk about beauty. This line could only describe a perfect woman if that woman happened to be white. Shakespeare doesn't say anything about this woman's race, but with her black hair and dark skin, we can definitely tell that she doesn't look like a Barbie doll.
And in some perfumes is there more delight Than in the breath that from my mistress reeks. (lines 7-8)
If people have stereotypes about how women should look, they have even more assumptions abut how they should smell. The ideal woman in a poem probably isn't supposed to smell at all, unless she smells like perfume. The idea of a beautiful woman with foul breath was probably as weird for people in Shakespeare's time as it is for us. So this is just one more way the speaker scrambles our expectations. Instead of a woman who smells like a rose, he gives us a woman who smells bad. Even though he's not being particularly nice, he's also refusing to base his love on some bland, worn-out poetic stereotype.
I grant I never saw a goddess go; (line 11)
What is a goddess but the most perfect, ideal, flawless woman possible? The trouble is, mortal women simply cannot rival immortal goddesses. If you compare a woman you know to a goddess, you're just going to wind up disappointed. So the speaker of this poem is just asking us to take a step back. He wants us to remember that it isn't really fair to compare someone you love to some fantasy woman that you've never even seen.