Having great tools. Excellent documentation. Being friendly to new users.
Yes, it's also a systems language without a runtime. But that's not the novel part. You could write horrors in C++ that approximate ML even without language support. There are eldritch libraries where some kind of pattern matching is done via generic lambdas.
The main difference is developper UX. Good tools, good error messages, quality of life. The novelty is making ML not painful.
Low level strong correctness was absolutely a novel part. In fact it’s exactly why many people glommed onto early rust, and why it was lowered on the stack.
Although learnability and weirdness budgets were also extremely novel in low level contexts which had been subsumed by C and C++.
> horrors in C++
Yes, horrors in C++. Half baked jerry-rigged and barely usable nonsense. Not an industrial strength langage with a reliable type system and a strong focus on safety through types.
Being memory safe without being managed is what makes rust a truly novel and interesting language for which it rightfully gets the hype.