I'd say the two big ones are:

- Mature support for async/await since 2012. Yes, Project Loom is coming, a decade later..

- Support for generic-aware value types (struct vs. class) and low-level features like stackalloc: very valuable for high-performance scenarios and native FFI. See for instance https://github.com/ixy-languages/ixy-languages. In comparison, Java doesn't even have unsigned integers. Yes, Project Valhalla is coming someday.

As well, debatable to some folks, but: properties (get/set); operator overloading; LINQ > Java streams; extension methods; default parameters; collection initializers; tuples; nullable reference types; a dozen smaller features