One of the things I found odd (or maybe just annoying) is how the Elm Architecture doc [1] keeps mentioning how you can cleanly model your code using modules. But the module system is about as primitive as you could imagine. The language would be much more expressive with ML's module system, and functors would relieve some of the pain of not having type classes.
[1] https://github.com/evancz/elm-architecture-tutorial/