Julia's syntax and language design is brilliant, but the biggest reason Julia has been my favorite language since I first learned to program is how astonishingly easy it is to use tools other people have made, and also to create tools for others to use.
The packaging system is just great for development, both as a user, and as a maintainer. I've been surprised by how easy it is to take stuff I've made and get it to work with a myriad of packages for the purposes I need. Multiple dispatch does wonders here too.
In the recent survey they said that 30% of respondents had contributed to a package. 30%! It's a super active ecosystem and the barrier of entry is low enough where I don't feel "imposter syndrome" as strongly as I have elsewhere--that I need to pass some threshold in order to properly contribute.