Here's a topic for discussion.
 
I'm not a lawyer or solicitor - legal advice is generally worth what you paid for it, this obviously costs nil.
 
So, as it has been explained to me and I understood it - 
 
A. If someone provides a service to you which was not carried out competently or sells you a product which was defective and you have attempted to resolve the situation without satisfaction - you'd be understandably annoyed and want to complain.
 
Now, if you did that on a television show, a radio broadcast or an internet forum - you'd be sharing your dissatisfaction. 
 
That would 'damage that person or companies reputation'. But would it be libellous? (written defamation, 'slander' is spoken) Well, it would have to be untrue, and more to that - it would have to be an intentional false communication. A lie.
It may also be in the greater public's interest to know, if i'm considering the same service provider and see that they do not live up to their claims it would be good for me to know if I was a potential customer.
 
The person or service provider may not like this public discussion and cry out 'defamation' - that's why if you make any such claim you need to know that what you are saying is correct and more importantly if challenged - you have proof of your claims. 
 
You'd find yourself in quite a bit of bother if you made claims that were untrue.
 
Do you think that major companies like it when they have messed up (ie the horse meat scandal in the UK) and the media gets hold of such a story? Or for that matter any other of the things shown on current affairs tv shows? 
If the claims are true it's not slander or libel. Sure the individual or organisation may lose 'reputation' but if its the cold hard fact thats the way it is.
 
So it's pretty damn important that you can back your claims up if called to. Have a look at the USA and UK forums, there's no shortage of service providers and sellers 'losing reputation' - and why is this still able to be read on the forums? Because it was most likely true and correct. If it was not true the person or company named would have a field day with the person making the post.
 
If someone expresses something in a public arena that does indeed 'defame' and it's anything other than the cold hard truth (with evidence!) the defamed party is well within their rights to come after the wrong party for financial damages. 
 
 
B. Now, if you find out secret and personal information about a person, just to be ridiculous i'll say that someone we know like to wear socks and sandals together. They are ashamed about it and do it in the privacy of their own home. It's something they don't want anyone to know about it.
 
You know about it and have proof of it. You make it public and the person is embarrassed and loses reputation.
 
It's not in anyone else's interest. You shouldn't do it and could find yourself being prosecuted for damages. 
.
 
 
The internets is not an anonymous slagging off playground. You are responsible for what you write. ISP's and all communications are traceable - unless you are one of those smarty types who do that stuff where you mask ISP's by using a server on the dark side of the moon or however thats done.
 
 
So all users of media including the forums, should have a clear understanding of the laws related to defamation and their personal liabilities. There are already cases against people that post defamatory comments on forums .
A key test is the comments include errors of fact and set out to create a negative opinion. 
If there's no 'error of fact' the negative opinion may indeed be what is created. If service or product is substandard for no reasonable 'reason' and it is not rectified the service provider or seller deserves the negative opinion.
 
 
 
So, anyone interested, know more than my limited scope or want to comment, fire away.