regarding online manners:
not saying HELLO, GOOD DAY, GOOD BYE, GOOD NIGHT, etc.. or not introducing yourself.
Sure, at times we are all super busy, stressed as hell, and saying hello or good bye doesnt happen. It happens to me aswell.
But some ppl NEVER do this on ICQ, email etc.
For me, having a conversation over ICQ or email is pretty much the same as I would in person or over the phone. So, I say HELLO or HI when I initiate a conversation, and I try and say good night/take care/or whatever at the end of the convo.
With close friends, it is a bit diffirent - as you can pretty much say what you want, any way you want. But in regards to doing business, or with people I dont know that well - I always try to make sure I start AND end the convo in a good way. If its been a while since I talked to someone, I (re)introduce myself a 2nd time just to make sure the other person knows who is contacting them!
I had people contacting me offering their products or services worth thousands of dollars, without even introducing themselves or their company.