I love to hear her speak (line 9)
It takes him more than half of the poem to get there, but the speaker finally says that he loves something about his mistress. This is a big turn in the poem, a shift from a list of criticisms toward an actual confession of love. That's not to say that everything changes, since he still admits that the sound of her voice isn't as beautiful as music (line 10). Still, we can see where he's headed now. He doesn't need perfection in order to love.
My mistress when she walks treads on the ground (line 12)
This is a really nice image of the simple beauty that the speaker loves. Why would you expect, or even want, your lover to float around like a goddess? What good does it do to compare someone to an imaginary perfect creature when the real living, breathing person is right there? Have you ever felt this way? Think about someone you love, whether a girlfriend or a boyfriend or a really good friend. Do you love her because she never makes mistakes, because her hair is perfect, because she's always clever? Or do you love him just as much (or even more?) for being goofy, or clumsy, or for looking less than perfect?
…I think my love as rare As any she belied with false compare (line 13)
Here's where the speaker finally just comes out and says it. Other poets might make up fancy comparisons for their lovers, pretending that they are as perfect as a goddess, as white as snow, etc. He refuses to do that; he simply loves this woman for what she is. This whole poem, while seeming like a criticism of this woman, is actually a parody of other poets. Still, we wonder if those earlier lines leave a bad taste in your mouth. If someone told you that your breath stinks, and then said he loved you anyway, would you be thrilled? Does the end justify the earlier lines? Is this what real love looks like?