In our opinion, the questions must be challenging but not too hard, interesting, accurate and relevant. We want people to look forward to their trivia night every week and to be discussing the questions for days to follow.
The only thing worse than questions that are too hard are questions that are too easy; just listen to the crowd groan when you ask a question that is too easy.
All crowds are different but we try to structure our rounds so that most teams will get 5-6 questions right, or at least should get them right. They are questions where the team might be scratching their head with the answer on the tip of their tongue and 'kick themselves' if they get it wrong. We then try to have 3-4 questions that are a little bit harder and 1-2 questions that are challenging to sort out the top placings.
As much as possible, we try and include questions that make people think and to promote discussion and debate within the team. An example might be: "How many James Bond movie titles have just one word in the title?" This is much better than a straight and boring questions such as Who is the President of America?