Anvilicious: Religion is LIES!!! A clever idea maybe but there are people who found devoting nearly all of the entire second half of the film to playing around with the concept to be very heavy handed. The promotional material marketed it like a soft comedy/drama about the hilarious hijinks one could come up with with a world without lies... only to realize that it's actually a commentary about how religion is dishonest?!
Mitigated by the fact that these lies were initiated by the protagonist, and with good intentions.
Given that Ricky Gervais, (who wrote the movie) is an atheist, this was probably the whole point. How you feel about religion sort of determines the message - is it a good result that came from a lie, or does it not matter that the result is good since it came from a lie? Or is it not a lie at all?
Fridge Logic: People aren't familiar with lying, but they evidently are with mistakes; the idea that, if there's ever a contradiction between people's senses or memories and what Mark says, that Mark couldn't just be mistaken, never seems to cross anybody's mind (even in cases where Mark states that somebody isn't drunk, despite physical evidence that they are, and despite how this recasts the assertion of the police officer that they are as a mistake).
It often comes off less like this is a world where no one lies, than a world where everyone is a complete Jerkass. Apparently not only can people not lie, they must say whatever comes to mind.
What an Idiot: In a world where deception is impossible, being a career criminal is possibly the dumbest thing anyone can be, never mind committing a crime of any kind. We get an example of how utterly stupid this can be with Mark's father.
Except, you know, Mark was probably lying about that, since he'd just discovered the advantages of doing so. The idiocy of being a criminal in such a world is what makes the anecdote about his dad so surreally humorous.