Cut to the quick, is the motto. The expense and eye-strain of online reading demands it. Be brief, but not bland or rude.
People are still people and rules of social awareness apply.
It's very easy to feel detached from what you are keying onto a screen. Know who you are writing for before you begin and plan accordingly. Use interior pages to unfold details and complexity. Cut every word that doesn't contribute. Make sub-clauses into separate sentences.