2. the thing they most do show: what their appearance strongly implies. —I think the poet's idea is that a beautiful person ought to have a beautiful, loving disposition.

4. cold: dispassionate.

5. rightly do inherit: i.e., make proper use of.

6. husband nature's riches from expense: keep the riches of nature (physical beauty) from wasteful expenditure or loss.

7. They are the lords and owners of their faces: i.e., they are complete masters of themselves.

10. Though to itself it only live and die: Even though it lives and dies only for itself.

12. outbraves: surpasses in splendor.   his dignity: its worth.