Here is some (fairly obvious I think) advice on criticism from Daniel Dennett:

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.

Good practical advice, I think.  Facebook comment from someone:

I don’t think anyone can accuse Daniel Dennett of being ‘kind’ when it comes to criticizing the things he doesn’t agree with. When it comes to religion and determinism, I think ‘brutal’ would be a more appropriate word to use. Wish I could think of some concrete examples, but the only one that comes to mind is his critique of Rick Warren in a TED talk.

Maybe he got through the first three without learning anything.