Having a distinction of if something does IO or not is quite important, and the fact that haskell has the ability to encode it in types is what other languages don't have.

Secondly, IO is just one monad. You can build a more granular one where you can separate filesystem, network etc, and encode this into types (so you know at a glance). You can't do this in most other languages.

What are the types of things that are worth being hung up on for a language for you?