I'm far more articulate online, I like being able to edit what I write so I express exactly what I want to. I'm pretty shy with people I don't know as well in real life because when you say something it can't be edited and changed. On the other hand I'm far loonier with people I know well.
I tend to use proper grammar and spelling and such on forums. As Brawny said, your words are who you are and that is what you are judged by. I don't find it annoying in particular when others choose to be "concise" though. But it can be unattractive when girls do it a lot. Very quick turn off.