Sentence:  “That's life,” she said, and buzzed off to keep her vigil, leaving me kicking myself because I'd forgotten to say anything about the <WSD>quality</WSD> of mercy not being strained. It isn't, as I dare say you know, and a mention of this might just have done the trick. 
Meaning of the word marked with <WSD> tags:  A property or an attribute that differentiates a thing or person.