1. Who is going to determine whether a given statement is a lie or not?
2. What about sarcasm and works of fiction? (And who (and how) determines whether a given post was intended to be sarcastic or not?)
3. What about a context? For instance, exactly the same wording of a message might carry a different meaning depending on the content of other posts in a thread. While I believe that this is somehow clear and obvious, what is not obvious is what makes a full context - is it just direct parent post, or whole thread or maybe even a larger part of a website? What if selection of context makes a difference (for example if I just use parent post as a context, the statement appears to be a lie, if I judge by the whole thread it possibly appears to be sarcastic). Now, who (and how) decides which context should be applied for analysis?
4. Transnational problems - which laws apply and how would you enforce it?
5. What problems would it actually fix? Lets say that from now on, I am not allowed to post on hackernews that Barack Obama visited my parents (he did not, it's a lie) - what problem it solves that I am not allowed to post that? (edit: formatting)