The best advice I have ever seen on how to criticize or disagree politely and effectively is here (too bad I haven't learned to follow it yet):

http://www.brainpickings.org/index.php/2014/03/28/daniel-dennett-rapoport-rules-criticism/

"How to compose a successful critical commentary:

1. You should attempt to re-express your target’s position so clearly, vividly, and fairly that your target says, “Thanks, I wish I’d thought of putting it that way.
2. You should list any points of agreement (especially if they are not matters of general or widespread agreement).
3. You should mention anything you have learned from your target.
4. Only then are you permitted to say so much as a word of rebuttal or criticism."

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PoserPro 2014, PS CS5.5 Ext, Nikon D300. Win 8, i7-4770 @ 3.4 GHz, AMD Radeon 8570, 12 GB RAM.