The worst kind of discourtesy is when someone says they will commit to something and then do the opposite leaving you stranded. Its too easy to do online and harder to do in real life.
No one is nice anymore but a lot of us don't care because we just get on get what we need and get out. Some might feel that's rude but its a fast world with little left for any kind of chivalry stuff. Sad but true.
The online world is no doubt a world of instant gratification and anonymity. Those two things make it easy to be rude. Even on this thread someone made a distinction between online and "Real world" so that is part of the problem. If it's not real you can do what you want with no consequences. I think that has spilled over into the non-online world sometimes in horrible ways like mass killings. Online discussions allow the dehumanization of people. I think fantasy roleplaying like ROK is a good use of the internet because then it's really not the real world. But lots of people are actually roleplaying when they are in a non-fantasy forum. The internet has actually led to the discovery of new mental health issues like covert narcissism, which is based on creating a persona who is perfect. We have to force our children off phones and into interpersonal interactions!
The difference between online world and real world is that in the online world because there is no physical/verbal contact between the two parties. Each side is hiding behind a keyboard and screen and they feel they can say whatever they want without even knowing how it will harm the other side.
That is also why a lot of people think that the newer generation who is online all the time is less good in direct human communication in person. They are less familiar with the culture courtesy as known until now.