Parentheses are now forbidden around named parameters. The following compiles in C#5, but not C#6

Console.WriteLine((value: 23));

Operands of is and as are no longer allowed to be method groups. The following compiles in C#5, but not C#6:

var result = "".Any is byte;

The native compiler allowed this (although it did show a warning), and in fact didn’t even check extension method compatibility, allowing crazy things like 1.Any is string or IDisposable.Dispose is object.

See this reference for updates on changes.