Lots of category variety, and I like that the difficulty level goes up or down with a right or wrong answer. Really makes you think! I also appreciate that each "lesson" has 10 questions and 10 categories, so there is only 1 question per category per lesson. This makes it an "even playing field", so that, for example, if you know a lot about sports and nothing much about history you have no real advantage over someone who knows nothing much about sports but a lot about history! This is a definite "plus" over some other trivia apps!!!