This article takes a detailed look at a successful, engaging product and breaks down why it is engaging across 6 factors. The author discusses the use of response speed, short term memory management, visual design and cognitive theory within the game to keep and maintain interest. It would be interesting to hear from the game developers if they used this approach, or if they were guided by their intuition.
This is a great way to think about products and how to engage users through system and visual design. The article will require a few minutes to read but is worth the investment.