In how many places does the concept of "waste" come in?
Granny Weatherall is a bitter old woman (although still likeable in many ways) who is angry about being jilted by the love of her life at the altar before their wedding. He failed to show up and she was never able to get over this.
The concept of waste is a constant in this story. Granny probably feels that much of her romantic life was a waste because she never got over her fiance jilting her, even though she very much loved her husband that she married later on. Also, because she was jilted again in death by God not giving her the sign she so desperately wanted to experience before dying. She keeps waiting for it and it never comes to her. The story ends with Granny dying unfulfilled in more ways than one!
It could also be viewed that Granny's attitude was a waste as well. Because she could not come to terms with being jilted instead of looking at it as making it possible to find a wonderful husband afterwards, she is wasting her own emotional energy and capacity. She also wastes time waiting for the "sign" before dying that she never encounters, so she dies bitterly instead of being at peace with her life.