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.
See also:
How to disagree by Paul Graham
which presents this disagreement hierarchy:
DH0. Name-calling.
DH1. Ad Hominem.
DH2. Responding to Tone.
DH3. Contradiction. (merely stating a contradictory opinion)
DH4. Counterargument (contradiction plus reasoning or evidence)
DH5. Refutation.
DH6. Refuting the Central Point.
Better Disagreement [1] from LessWrong adds:
DH7. Improve the Argument, then Refute Its Central Point.