After skimming over this long thread, I'd like to make one more point which I haven't seen mentioned yet:

A world without implicit contracts does not necessarily lead to chaos and endless civil war.

In his book Practical Anarchy Stefan Molyneux describes how such a hypothetical society could function and thrive.

The idea is that there are several dispute resolution organisations (DRO) competing (and cooperating) with eath other.  There are no implicit contracts. There are only explicit contracts with DROs.  In practice, almost everybody would end up being a member of some mainstream DRO.   

So let's take some examples that Onarchy has used, and see how DROs would resolve this.

1) Someone who produces kiddie porn. 

This one is easy. Since kiddie porn is almost universally condemned, this person would amass a terrible reputation, and soon he would have trouble finding a reputable DRO prepared to take him as a member.  Without a DRO to protect him, someone might just "accidentally" murder him as soon as he is forced to leave his property to find food, because he has no explicit contract with 99.99% of society protecting him against murder. 


2) Defamation  and Threats

In a world where anybody can commit the "crime" of defamation against anybody without fearing physical retaliation, baseless defamation and relaliatory defamation would soon spiral into a "flame war", and nobody would take this kind of information seriously anymore unless it's backed up by evidence.  Also, people would no longer believe information outside their web of trust.  If someone defames a person inside their web of trust, and the published information turns out to be baseless, or a cruel breach of privacy,  they would lose hard earned trust from a lot of people.  That would act as a strong deterrent.   

In short, society would acquire more healthy skepticism, and paradoxically, there would be less defamation even though it's not explicitly illegal. Implicit libel+slander laws are similar to "consumer protection" regulation leading to passive, gullible consumers, who dishonest companies find easier to rip off. 

3) Being murdered because I'm not wearing a sign "don't murder me".

Again, since murder is almost universally condemned, almost every DRO would have a clause against this, and reputable DROs would have reciprocal agreements allowing to "extradite" murderers to each other.  The putative murderer would be wise to assume that a person on the street has a 99.99% chance of being protected from murder by their DRO, who would hunt down the murderer.   Ok, this model still doesn't protect against "crime of passion" murders but neither does the statist, implicit contract model.