In "Sonnet 130", how does Shakespeare describe the lady he loves?
The best and most simplistic answer to this question is: honestly. Shakespeare is honest about his love, and describes her as an average person. He says here eyes are "not like the sun" - meaning they are not overly bright. He says that her is black - "black wires grow on her head". He says that her breasts "are dun", meaning that they are not pure white but a more faded and average skin tone. In Shakespeare time, pale white skin was the most attractive; browner skin suggested a hard-working or outdoor life.
Going on, Shakespeare explains that her breath is not like perfume, and her voice is not like music. Again, he is being honest. He is saying that she is not perfect. Shakespeare does this in reaction to so many love poets that over-glorify their lady loves. The metaphors used by the other poets suggest that the woman are like goddesses and are meant to be worshipped. Shakespeare refuses to make that comparison here. He actually refers specifically to that:
I grant I never saw a goddess go
My mistress when she walks treads on the ground.
However, his argument is that his love is even more pure than the love of all those flowery poets, because he is able to admit the flaws and limitations of his mistress and love her all the same.
And yet, by heaven, I think my love as rare
As any she belied with false compare.
He is implying that other poet's compare falsely, and so their love is not as rare or as pure. His - being based in reality - is the true love.
In "Sonnet 130," Shakespeare describes the woman he loves as a real person instead of exaggerating her beauty.
At first, his description seems almost insulting. He says that her eyes are dull -- not bright like the sun. Her lips are more pale than coral. Her hair is black wires. There are no roses in her cheeks. Her breath smells. Though he likes to "hear her speak," he knows her voice isn't as beautiful as music.
He says, "I grant I never saw a goddess go; My mistress when she walks treads on the ground." Shakespeare is saying that she's not a goddess. She's a mortal woman and he recognizes her flaws and shortcomings.
In the end, Shakespeare makes it clear that loving someone when you see their flaws is what matters. He says that it's rarer to love someone with full knowledge of their faults than to idealize someone and love them for what they aren't. His love is the truer love because it's the one that's real.
This sonnet is very different than many other love poems. Instead of idealizing his love, he humanizes her.