In theory, honesty would be the most humane way to end a relationship. But in reality, no one wants to hear an excruciatingly long explanation of why he or she has become unlovable. The idea here is to give an outline on why you want out, not a 10,000 word thesis. Explain enough to give a sense of closure; leave out enough to keep from being cruel.
A gentle outline looks like this: I don't think we're compatible enough to have a future together. We lack [major relationship issue] and we don't see eye to eye on [fundamental values that are different].
A brutal thesis looks like this: I could never marry you. You'd be a terrible mother. I hate the way you [a long list of little, arbitrary things she does that annoy you]. I think I can find someone better/more attractive/intelligent/sane/interesting/freaky in bed, followed by more needless details re: why she is inferior to the awesomeness that is you.
A breakup shouldn't feel like an attack on the other person's character. It should be an honest assessment of how and why your feelings have changed, without resorting to verbally assaulting the other person by enumerating their faults. That's not honesty; that's emotional terrorism.