- Passing not evaluated block as argument ("by-name arguments". Allows to develop better DSLs)
- There are few libraries written with macros, impossible to implement in a language like c# or f# (MacWire etc.)
- No not-nullable types (this is a huge one)
- Not whitespace sensitive (curly braces everywhere)
- No type inference for arguments and, sometimes, for result type (signatures involving generic types may be really hairy)
- No compiler warning on implicitly discarded expression value (possibly wrong code like arr.map(x => x * 2); arr.map(x => x * 3). The result of the first map is discarded silently. In contrast, F# forces us to write arr |> Array.map (fun x -> x * 2) |> ignore; arr |> Array.map (fun x -> x * 3).