On the other side
It's too overwhelming to remember that at the end of every computer is a real person, a lot like you, whose birthday was last week, who has three best friends but nobody to spoon at night, and is personally affected by what you say.
Probably the biggest problem with communicating online is the lack of context. When online you can’t see facial expressions, hear the tone on their voice, or anything. All you get is a blurb of text.
I know I can be difficult for others to read at times, because I can be quite short and terse. And I’ve certainly had encounters like Derek Sivers pointed out in his post with doing support.
Since reading that post, I’ve tried to remain more conscious of that when working with others. But it is also important to keep it in mind when you receive emails, IMs, comments, or other messages. It can be easier for them to come off the wrong way and you just need to realize it might not actually be meant how it sounds.