What do you consider the hallmarks of a good developer experience? How are you fostering that in your org?

Its to remove the friction, accessibility to the tools the developer prefers. 

Its all about setting up the framework for success. Like any employee you need to work out what motivates, inspires and causes frustration and build your onboarding and engagement program around that. We have a 100 day onboarding program where through strategic actions we take to uncover that at an individual level however usually it ends up being quite similar per developer as it takes a certain type of person to be a skilled developer