Hot code reloading. It was one of the best decisions I made when I added it to a hobby project I've been working for the last six years. Most of my development time for this project is spent adjusting code while the app is running. The feedback loop is incredibly tight and the most engaging of all of the projects I've ever worked on.
I'm working on extending auto reloading to all of the assets in the project because I know that tight feedback loops are that important.