2. gaze: object of the gazes of others.

4. unfair: make ugly.  which fairly doth excell: which (now) excels in beauty.

6. confounds: destroys.

9-10. Then were not summer's distillation left / A liquid prisoner pent in walls of glass: i.e., then, if it weren't for the distillation of summer flowers into a perfume enclosed in a glass vial.

11-12. Beauty's effect with beauty were bereft, / Nor it nor no remembrance what it was: i.e., both the effect of beauty (such as the adoring gaze of others) and beauty itself would be lost, nor would anyone remember what it was.

14. Leese: lose.