Nothing about saying, "Thanks, but this won't be accepted until X, Y, and Z are fixed," precludes having a conversation. By all means, if you think X is not a problem, I would expect a response along the lines of, "I don't think X is a problem because insert mitigating factor here." Or, perhaps, "I don't understand the issue with X; could you explain?" In other words, allow directness to beget directness. Enumerating all the possible ways you might be wrong, just in case one of them is actually true, seems silly to me.