'Nuance' is from French and means a delicate or subtle degree or shade of difference. Those who accuse Kerry of fondness for it presumably mean that he is too ready to be nit-picking or finicky in his approach to things.
Yes, that Kerry won't give a simple yes or no answer, but insists on defining the terms, explaining the situationunder which it's Yes and when it might be No, etc.
As opposed to our fearless leader, who answers from his gut and refuses to consider that he might be.... you know.... the W word.... (((wrong)))