>>103227729
Hard to pick just one.
>Significant whitespace (Haskell, Python)
>List comprehensions (Haskell, Python)
>Currying (Haskell)
>HM type system with type classes (Haskell, Rust)
>Mixfix function calls (Smalltalk, Objective C)
>Native bignums (Python)
>Hierarchical namespaces with dot notation (any language worth using)

One thing I have learned is that syntax sugar is more important than it seems. If in one language a feature is #foo and in the other it's Make_Octothorpe(foo), this will affect the entire coding style and philosophy of both languages.

AST procedural macros

>>103227975
>Hierarchical namespaces with dot notation
this one is nice, but the same syntax for struct dereferencing is also mandatory. Idk if both of them existing within a single language that also have type inference but it's possible.
>>103227981
highly mandatory. language without should have stopped existing in 2000

the state of /g/