The ecosystem. The worst thing about Java and best thing about C#.
C#: 90/10 writing code Vs. fighting with the tools/environment/library
Java: 10/90
Is that cheating? Likely. But it is still true. C# isn't a very much better language than Java. But the tools/environment/documentation is terrible in Java.

I think this code example sums up both C# and Java perfectly:
if(args[0] == "test")