From our perspective in the United Kingdom, if the behaviour which is currently criminal is to remain criminal and also capable of prosecution, we consider that it would be proportionate to require the operators of websites first to establish the identity of people opening accounts but that it is also proportionate to allow people thereafter to use websites using pseudonyms or anonymously. There is little point in criminalising certain behaviour and at the same time legitimately making that same behaviour impossible to detect. We recognise that this is a difficult question, especially as it relates to jurisdiction and enforcement.
The report notes that the findings are "tentative" and that these recommendations might possibly "be an undesirably chilling step towards tyranny," but they don't seem that concerned about it, or they wouldn't have made the general recommendation in the first place.There is a long list of problems with such a proposal, beyond the obvious questions of how you would possibly enforce it and what the various chilling effects would be. But let's take it one step further and note the fallacy of the very premise made in the report: that without such requirements it is "impossible to detect" who did an action online deemed to be illegal. We've been dealing with this issue forever. A decade ago, we reported on the various freakouts over open WiFi and how it would "allow" anyone to commit crimes online and make it "impossible" to find them. And yet, time after time, we noted examples of basic detective work allowing police to track down the criminals.
Yes, without being forced to first identify yourself, it might make the police work a bit more difficult, but never impossible. Take a similar situation in the physical world. Anyone can walk into a store or a bank and hold it up. And they can do it without identifying themselves at the door before coming in. It happens all the time. Police have no official identity to work with, but they do have other clues -- fingerprints, video, photos, the clerk's memory -- to work off of and can piece together who committed the crime. The same is true of people online. Even if they don't identify themselves upfront, they frequently leave plenty of clues that allow law enforcement to figure out who they are.
So the very premise that this is somehow necessary is pretty much eliminated. Then combine it with all of the downsides that we already know about: chilling effects, the end of important anonymity, potential privacy violations and leaks and more. What you're left with is a horrible idea all around.