Languages have features/constructs. It's better to look at what those are. And far more importantly: how they interact.
Take something like subtyping for instance. What makes this hard to implement is that it interacts with everything else in your language: polymorphism, GADTs, ...
Or take something like Garbage Collection. It's presence/absence has a large say in everything done in said language. Rust is uniquely not GC'ed, but Go, OCaml and Haskell all are. That by itself creates some interesting behavior. If we hand something to a process and get something back, we don't care if the thing we handed got changed or not if we have a GC. But in Rust, we do. We can avoid allocations and keep references if the process didn't change the thing after all. This permeates all of the language.
Languages have features/constructs. It's better to look at what those are. And far more importantly: how they interact.
Take something like subtyping for instance. What makes this hard to implement is that it interacts with everything else in your language: polymorphism, GADTs, ...
Or take something like Garbage Collection. It's presence/absence has a large say in everything done in said language. Rust is uniquely not GC'ed, but Go, OCaml and Haskell all are. That by itself creates some interesting behavior. If we hand something to a process and get something back, we don't care if the thing we handed got changed or not if we have a GC. But in Rust, we do. We can avoid allocations and keep references if the process didn't change the thing after all. This permeates all of the language.