Having learnt a few different languages over the years I have a shortlist of my favourite features across different languages (note I have included a few things that are not strictly language features, but are commonly used libraries that I think are just great!) - I will list my own thoughts below, but I thought it would also be a fun question to ask the community!
So here's my list in no particular order:
What are your thoughts on these features? Would you include them if you were to write a language? What else would you add and why?
Edit: one that is really useful, some would say absolutely key, in modern web development, is native strong-typed JSON serialisation/unserialisation - ie being able to convert easily between object and JSON representation without having to manually map the internal fields, especially where nesting is involved. C# has a great implementation utilising generics for the "to" object type when deserialising - the equivalent serialiser is pretty good too, with easy support for custom converters for things like string representations of enums, etc