It's not so much that there's a single killer feature, but that it's able to pull from everything that emacs already does well, and interoperate between them. A

It's not so much that there's a single killer feature, but that it's able to pull from everything that emacs already does well, and interoperate between them. A common use-case for me is using one language to process some data then passing the result to another tool (e.g. gnuplot, graphviz) visualize the results.