The teammembers: The quality of a piece of software is directly portional to the amount of communication between the developers. The human relationships are critical to success...of any project...but most important in new emerging markets like serious games. All teammembers MUST be in line and supporting the vision.
The client: listen to every bad suggestion...very deeply. Figure out what the real question is before fighting with the client. Care about what the client cares about. Think the way they think.