My colleagues have answered this very well. I'd like to add a few comments.

Summer traditionally represents the time in life when we are fully blooming. Spring is the virtuous youth. The metaphor he is playing with is the traditional notion that we all live the seasons of man, and that we have the most promise in the spring and are at our hottest (ripest) in the summer. As my colleagues have pointed out, the metaphor breaks down: it is too hot in the summer, while she is temperate. He can't compare her to a rosebud either, because they are vulnerable and apt to be destroyed. These two metaphors refer to physical and spiritual qualities: the summer is the full realization of her beauty (and also, possibly, includes a sexual awakening); the spring speaks of virtue (buds=virgins) as well as promise. Just as the summer is too hot to work as a metaphor, there are problems with the use of the spring/bud metaphor: rough winds. The problem is that in nature, no virtue lasts forever, and every promise of perfection ends in destruction. So the nature metaphor is rejected. He also uses the metaphor of Death's shade to show the possibility that she could be eclipsed by death, that death could hide her or that she would fall into his shadow and be forgotten. The final metaphor compares the eternal spirit (or essence) of a person to the eternal power of the written word and, implicitly, the poet to a god: she lives forever in lines that he writes, after all.