Okay, sorry, I assumed you were still talking about libel.
Let's cut to the chase, of the statements posted about Mark's article, which are libelous and under what country's law, and why is it likely that that county's law would be applicable?
Dean - see John's response above for examples. As to where and when? In New South Wales according to Defamation Act 2005 - Section 7 (2) "Accordingly, the publication of defamatory matter of any kind is actionable..."
So that basically gives the general scope - everything. There are then special exclusions (certain corporations, dead people, people speaking under parliamentary privilege, magistrates and judges in court and so on).
To bring a defamation action in New South Wales you need three things.
1. To prove the material was published (the definition of this is much expanded in the 2005 Act, particularly with the removal of the distinction between libel and slander).
2. Next, you must be identified/identifiable as the aggrieved person. If you can't reasonably determine that you are the person in question, you can't take action.
3. The material must be defamatory which typically means it:
a. exposes someone to ridicule
b. lowers the person's reputation in the eyes of the community
c. causes people ot shun or avoid the person
d. injures the person's professional reputation
Of course there are defences, not least of which is truth.
So, calling someone a charlatan, for example, could certainly lower the person's repuation in the eyes of the community. Since said person is clearly not a charlatan (check the meaning of the word) and even in a broader context of that definition there is no means by which it could be considered truth, then there exists a basis to persue an action under the tort of defamation under New South Wales jurisdiction. All Australian states and Federal jurisdictions are the same. Most Commonwealth or English Law derived or based jurisidictions have similar provisions (the US being a notable exception).
So, yes, John's comments are perfectly valid.