Since no lying means no imagination, most of the consumer products in the film are boring and unimaginative. The computers, monitors/televisions, stereo equipment... even the office decor. Humanity invented these things out of necessity, without any flair. It's more subtle than the advertisements in the film, which are more prominent in their mundaneness, but it goes along the same concept.
As to why there isn't any religion. Go to your nearest bible and read about what Jesus said. Most if not all of his lessons are stories about hypothetical situations, metaphors, or other fiction. Religion is told through stories and since no one can even think about falsehood spreading the word becomes rather hard.
Also, religion can't be proven either way. Since no one lies, saying something you don't objectively know to be right must be very hard.
It seems like this would fall under the things people believe to be true, however. People who are devoutly religious certainly don't believe it's lies (nor are atheists lying when they say it is, or at least unproven). Assuming you take a purely secular view, it's still not "lying" to surmise that there is an intelligence behind events, that the essence of a person doesn't just fade away after death, etc. The film has a plot hole here I think.
In a world where partners select each other for genetics, marrying Mark makes perfect sense, since he has a superior ability, which turns out to be congenital.
We can assume that fraud and other acts of deceit are unknown, and therefore not criminalized. Therefore, Mark never does anything illegal.
Since everyone is gullible that they'll believe anything anyone told them, despite all evidence to the contrary, schizophrenia must be contagious. The crazy person says something crazy, everyone believes it, they repeat it, and so on.
Schizophrenia is not simply believing "something crazy" is true. Although, this does bring up the question of how people would react if they realized Mark was saying things that aren't...
If everyone believes in a happy afterlife and the world of the living is cruel to anyone who isn't good-looking, what's stopping the suicide rates from going up?
One that's been asked to death but let's ask it again here: In a world where everyone is brutally honest all the time, why hasn't the human race wiped itself out?
Maybe because, for all their jerkassery, even without the protagonist's lies to keep them in line people aren't that bad.
You could argue that for some things at least, it would improve the world-no CE Os defrauding millions, for instance, crimes solved by simply asking suspects if they did it (as is seen in the movie), etc. A baby will cry because they have realized that crying gets them attention. And, as any parent knows, children will lie to get out of trouble or hide that they were doing something they shouldn't have been. This suggests, and is further proven at the end, that lying is a genetic trait in this world. This begs the question if Mark was simply the first mutation to develop the gene or if it is a recessive gene many have that just hasn't been activated.
In Real Life, there are some physiological safeguards against lying (blushing, etc) which seem to be carried on by evolution. This movie just take them one step further.
Which doesn't even make sense when you think about it. Wouldn't it make more sense for evolution to favor good liars?
One can also wonder how thieves can ever make a living in this world. As shown in the scene with Mark's father, catching a thief is trivial.
This brings up another question: why can't suspects just refuse to answer? Of course, if everyone only tells the truth, one might surmise that torture is a common interrogation method, since it really would always work in this world.
If Mark could mention aliens and not have to explain what they are to everyone, that means that no one has invented them. Therefore, do aliens exist in this world?
Wondering could still be possible. Everyone is just honest that they don't know if aliens exist or not?
If Mark is automatically believed when he gives a different number for his bank account than what he has in there, does that mean that humans in this world all have perfect recall and don't make honest mistakes? Or see something and initially mistake it for something else? Did no animal in this world ever evolve Mullerian or Batesian Mimicry? Do animals like cats not use threat displays to make themselves appear larger and more formidable than they really are when they see a predator?