One of the biggest things was the addition of Span<T> and Memory<T>, and their usage in underlying types within the .Net framework. While C# always had the ability to use raw pointers, these two types allow you similar performance characteristics without unsafe blocks (and frankly are near idiot-proof instead of being a giant foot-gun with pointers to managed types).

https://docs.microsoft.com/en-us/dotnet/standard/memory-and-...