Online spelling & grammar are two Big Things for me – and have been since my earliest days online (and that’s a long time ago).
My own attitude to it used to almost embarrass me and I’d actually apologize to people whose spelling was winding me up… now how mixed up is THAT?
Then I realized… spelling and grammar are so very important online, even in casual situations like IM, because they’re all you’ve got to go on when you’re building a relationship. If I’m reading a technical forum and someone’s offering advice but can’t spell then I devalue their input. I assume that, if they don’t have the knowledge to spell words correctly, then maybe they don’t have the technical knowledge I’m looking for. If they don’t care enough to spellcheck their post then maybe they don’t care enough to check their facts. I make a judgement of someone’s intelligence based on their use of the written word because I don’t have very many other (maybe more accurate) cues to work with. But you know all that.
Now if it’s someone I actually know personally then it’s not so important – I’ve met them, I have a relationship with them, I’ve judged them on a hundred other factors which don’t come across in a couple of brief online sentences. I understand how busy they are and even the odd their/there or misplaced apostrophe isn’t going to make me forget how smart they really are!