Good projects integrate work and play

Play here refers to general play, and often solving problems but especially problems vs puzzles|solving puzzles can be a good example of play.

For example, programming is playful when it is done in a comfortable environment. When a developer knows the space they are working in and is not stressed by external factors, then the act of programming

Another way of putting it would be to say that good projects involve both work and flow, and great projects involve both work, flow, and general play.