1 It mustn't sound too easy (What is the capital of France?)
2 It mustn't sound too difficult and therefore unfair (What is the capital of Chad?-unless Chad is in the news.)
3 It shouldn't permit a variety of answers (What is the county town of Wiltshire? Trowbridge is the administrative centre, Wilton the ancient county town, Salisbury the cathedral city, Devizes the traditional home of the county regiment, while Warminster and Marlborough also have some claim to the title.)
4 It is safer if it doesn't provoke controversy (How many counties are there in Ulster? The answer is six if you take Ulster to mean modern Northern Ireland; nine if you take it to mean the historic Irish province.) 5 The answer must be researched and double checked - no matter how obvious it seems. An earlier edition of Brewer's Dictionary of Phrase and Fable (an otherwise admirable source of questions) stated that a yellow flag means a ship is in quarantine. In reality, a yellow flag means "I have a clean bill of health and require clearance."
6 When you hear the answer, it should seem to have been a fair question ("Oh, I'd forgotten that" or "That's really interesting" are the reactions many a question setter dreams of provoking.)
7 Ideally, the question master should be able to pronounce all the words in the question. The setter may not have much control over this point.