Having a game that's easy to modify also spurs player interest. Be it the ability to replace models, add sounds or just simply change a texture or font. It allows the community to make what they want, and a large majority of game developers started as mere modders.
Having a developer console in the game is simply amazing. It can free a player's experience by letting them change settings they couldn't before, and change a few values to see how a game can run with things like low gravity or double damage with certain weapon.