In my time as a producer at CCP Games, the project I am most proud of is the Photon UI project. We had to devise a way to upgrade the old UI (which suffered from multiple UI styles implemented at different times in the past, performance issues, and usability concerns), while considering the heuristics and (often conflicting) playstyles of new and seasoned players.
This case study details our approach and the resulting outcome of our year-long project.