Adding to others, with disclaimer that I haven't built a large application yet so these are relative newbie comments:
1. An amazingly helpful compiler. The best I've come across for identifying problems, describing them in a meaningful way, and suggesting how to fix.
2. A wonderful synergy between language and architecture. Various others have copied Elm's model-view-update architecture (React, F# SAFE) but as they're libraries, they don't feel quite as well-integrated.
3. An excellent, helpful community.
4. A BDFL in Evan who is focused on considered design and careful evolution over "move fast and break things". Some will (rightly) challenge if this is good: Elm 19 broke a lot in 18. For me personally, I much prefer Elm's thoughtful, well-considered evolution over the volatility in the wider javascript ecosystem
5. Some nice libraries. Elm-ui, for example, provides a different way to do lay out (no CSS). Again that's not for everyone - but useful if you're not a CSS guru.
6. Strong static typing with powerful type inference.
There are definitely downsides. The full-on, no-compromises immutability means there's some overhead in dealing with e.g. http request/responses or js interop. But there's corresponding value in the resulting guarantees in the code.
As a newbie I'm really taken with Elm; it's a refreshingly stable and integrated enclave in the otherwise volatile front end space. YMMV of course
A strictly typed pure functional language that generates JavaScript. That, along with a clever compiler, makes them claim that apps built with Elm don't have runtime exceptions.
