The best form of communication is ultimately based on the amount of shared context (Eg: acronyms), so it’s silly to create rules per the communication medium — slack, email, HN comments, etc. Even within a medium, a slack DM to a friend has very different shared context from a message to a company-wide channel. Being mindful of that, and communicating respectfully, regardless of the actual medium, is likely to be far more useful. Nothing special about Slack.

For me, respectful written communication also includes proper punctuation & capitalization — since it makes reading easier.