I think the problem with harassment is that it is almost impossible to define what it is (i.e. what are the boundaries between harassment and a joke or harassment and an annoyance). Since there are no real firm definitions it is really a subjective thing which makes it hard to police.

My personal opinion is that there is online harassment, something that makes an individual uncomfortable enough for them to have to change or modify the way they live their life (what websites they visit etc) could be considered harassment (this is only my opinion). I don't think anyone has the right to make others feel uncomfortable or unsafe, be that when they are online or otherwise.

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My observation is that there is a hell of a lot of people who in the online environment are more then happy to bully or harass others because they can hide behind the perceived anonymity and the fact that they are not located in close geographic proximity to the person they are bullying or harassing.
If I was to be brutally unkind my guess would be that a lot of these people were probably physically bullied or harassed in "real life" and now take this out against others online because of the relative safety and anonymity that the internet and online channels provide for the bullies/perpetrators of harassment.
I have seen it a number of times in the past on this site, individuals who post with the underlying tones in their post of "Why am I wasting my time replying to this rubbish, this person is so stupid and doesn't belong here, and I am definitely smarter then you, idiot"
But I am not targetting this site, I have observed this on many forum type sites.

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Lets say that individual A feels bullied (i.e. made to feel uncomfortable) by individual B online. Perhaps this bullying should be considered harassment only after individual A informs individual B that they feel harassed/bullied. If individual B continues to do it then I think this is harassment because B is aware that A is uncomfortable yet chooses to continue.

This is good in theory but unfortunately some people are not comfortable telling others how they feel or if they think they are being bullied so in practice it may not be appropriate.

I don't think there are any real "answers" here but I think it is a good and healthy debate