As Dr Farid has already given the 2 important cases, i would like to add 3rd one. I met one reviewer who asked on what i have not done/ included in the particular submitted manuscript but not on what i have done!!! In fact asking for additional experiments, although my manuscript already contain enough experiment for an acceptable level. I disagree to add but respond politely with reasons and my manuscript got accepted and published. So, politeness should always be present either you agree or disagree.
I believe reviewers do a great job on our manuscripts. I always commend the work they do, especially, when the manuscript is submitted to a top journal.
One important think to remember is that, a Reviewer performs voluntary service during reviewing. Whatever he/she does, is for the betterment of the scientific community. Therefor, he/she should have been thanked for providing such voluntary service. However, he/she may not understand a particular portion of the publication. But it is not his/her fault. It is the fault of the authors, since they are unable to express their research work to the Reviewer, in broader sense the whole scientific community. Therefore, every time, Reviewers' comments should be addressed properly so that, a good publication can be obtained.