I think it's important to differentiate between a review and a rant.
If you have a bad experience and write a critical review, explaining why the service was below expectations then you should be fine. If you rant about a bad experience you have to be careful to construct your rant like a review, and explain why you feel so negatively about the experience. If you clearly explain what they did wrong, then you generally should be fine.
The basic rule is that when ranting, you have to tell it how it is. No matter how annoyed you are, you can't slag off a brand, or make it sound worse than it actually was, as that is deformation. Things such as personal attacks against staff, witty comments or asides, or defamatory phrases against the brand could potentially put you in the wrong.
I suspect this article is really targeting extreme cases, where a rant has gone out of control and someone has slagged off a business.
A good test is to ask yourself what a lay person reading the rant for the first time would think. Is it a well reasoned criticism, or is it deformation?
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The article is correct to point out that you may be personally liable for what you write online. If you write a bad review for a newspaper, the editor of the newspaper will have to approve your article, and the newspaper will bat for you in any legal proceedings. Many websites make users agree to terms & conditions which limit or exclude the liability that the website takes for what you write. That is, you are the 'publisher' of what you write on the website, rather than the website.