+ 1 for hot code reloading. Don't know if it's possible, but that would be phenomenal given that development cycle speed is a pain point for compiled languages
Another point of interest along the same lines would be something like Ruby's Rails Console https://pragmaticstudio.com/tutorials/rails-console-shortcuts-tips-tricks
For those who are unfamiliar, it gives the user access to variables and methods within a running server along with a few other features, so you can do things like test methods, run synchronous ORM queries, and hit routes and receive results. For example:
>> movie = Movie.first // retrieve the first entry in the Movie table >> movie.title // access fields => "Iron Man" >> movie.rating => "PG-13" >> movie.director => "Jon Favreau"