There are no "exact" limitations. Rather it has to do with the Bern convention allowing nations to restrict copyright protection in "special circumstances" that don't "unduly" interfere with "normal" exploitation of the work by the rights-holder.

In other words it allows nations to pass laws relating to what constitutes fair use. I would assume that absent this sort of thing the Bern convention would allow the WTO to impose sanctions on any nation that decided we were right regarding Oracle v. Google, Sony v. Connectix, etc.

Read the article. It's said that the main lasting problem is that no one actually said what the limitations are. So at the end of the day this could be all bullshit... but the author of the article hopes that the limitations will live to the expectations.