That is, the now-notorious case John Fairfax Publications Pty Ltd v Gacic. And hasn't there been a lot of ink spilled over this in the last couple of days? I particularly like the hyperbole of The Guardian's précis "Australian court opens the way for restaurants to claim damages over unfavourable reviews".
At best it would "open the way" for Australian plaintiffs. Given that English defamation law gives defendants much more latitude, it's not likely to have much impact there. And given that England is pretty much the only lively defamation jurisdiction in the world, here and there is about the extent of any legal shockwaves.
Furthermore, the analyses of the case so far (I do use that term lightly - there's not much good legal reporting about) haven't been very incisive. The case concerned the Defamation Act 1974 (NSW) - which is now defunct, since the uniform defamation laws came into effect. I haven't gone through the High Court's decision with care to consider if and how the case would stand under the new Act, but it might be different. NSW had a somewhat idiosyncratic system.
Secondly, although lazy journalists have picked up on the odd judicial remark about business reputation being susceptible to the various tests that an imputation is defamatory, the High Court's decision seems to hinge on more technical aspects such as whether an appellate court could substitute a jury's verdict. If anything, the case is more about the powers of the NSW Supreme Court. I.e., really, really dull. The High Court was just deciding on the basis of the legal framework, not on defamation law generally.
The High Court's decision is NOT about whether you're verboten from writing critical things in a restaurant review. If the lazy journalists had read the headnote of the decision, they'd realise that. But not much of a headline, I suppose.
I won't say anything with great certainty, because I've only skim-read the judgments, but I don't think this case is worth people getting their knickers in a twist. It's crummy for Matthew Evans (okay, Fairfax, since they're footing the bill) but I doubt it'll "open the way" for anything.
(ETA: I feel I should apologise on behalf of my law degree for my own lack of critical insight into the case. I even studied defamation law, so I should be able to say something a bit cleverer. Oh well!)