See http://mutanatum.com/posts/2017-01-12-Browser-FP-Head-to-Head.html for an overview.
Great Performance - Elm has for a long time maintained its leading position in performance benchmarks against the most popular web frameworks.
Enforced Semantic Versioning - Elm's tooling is built around semantic versioning.
Small Assets - This has greatly improved in the latest version 0.19
Many actively maintained packages can be found on the official Elm packages page.