The guidelines say to write as if you were face to face to a person. You wouldn't probably mention those nasty things, or at least you try to hide it behind constructive criticism.
Take this one step further: For everything you write, imagine the utmost authority on that topic reading your post. A blunt example: A rant about Python syntax. Imagine Guido van Rossum reading that during his coffee break.
Do not post if you can not add anything to the discussion. Assume your debater is smarter than you and knows more on the subject. This is still HackerNews. Ask someone if they have won the Putnam prize, and you may be unpleasantly surprised. [1]
Take this one step further: For everything you write, imagine the utmost authority on that topic reading your post. A blunt example: A rant about Python syntax. Imagine Guido van Rossum reading that during his coffee break.
Do not post if you can not add anything to the discussion. Assume your debater is smarter than you and knows more on the subject. This is still HackerNews. Ask someone if they have won the Putnam prize, and you may be unpleasantly surprised. [1]
Read and practice: http://www.paulgraham.com/disagree.html
Use your spell-checker. Write shorter sentences to avoid grammatical errors. Nobody is too smart for short, simple sentences.
[1] https://news.ycombinator.com/item?id=35079.