With all due respect to the commentary above, I tender my reading:

Yes, it's true, her eyes are nothing like the sun: they aren't twin fusion reactors spewing forth all manner of electromagnetic rays. Her hair is not like black wires, which are stiff and cold, and coral is rough and has lots of little holes with little animals in it.
Snow is white, cold and even if she painted her breasts white, they wouldn't look like gentle swells that myriad crystals form. Her cheeks aren't the blueish magenta of a wild rose. No, her breath isn't perfume, she may speak and walk, but she doesn't make the sound of a viola di gamba, nor does she levitate. 

This isn't because she's ugly. Think about how ugly and artificial and downright scary she would be if she were all these things. If she actually spoke in music, she'd be hard to understand if she asked for a kiss. She floats over, her lead blindfold ready. The little coral animalcules wave encouragingly…She breathes some Chanel No. 5 your way...

She isn't anything at all like that. She's better! Even in Shakespeare's day, all these similes had been thoroughly mined as to be essentially meaningless. Like the flattery of the two evil sisters in King Lear, they were the stock in trade of anyone writing a love poem: to say otherwise was to invite the kind of reaction that we find in the WU above. Like Cordelia, however, the last two lines aren't flattering lies, they're true. It's important that this comes very late in the Sonnets, this is, like Lear, the work of a mature man.