As someone who has worked with both Java and C#, I would say while Java might have been "good enough" it's lacking when compared to C# across the board. Microsoft has consistently improved the language ahead of Java, with new technology like generics (or properties, but you could argue about that one). Also, the development environment for C# is much better than whatever you can cobble together for Java.

I think much of the difference comes from the nature of the language designers and the timing of the language's creation. Joy had the handicap of going first, and Java has some choices that reflect that. Hejlsberg got to look at the weakness of Java to make C#. Overall, I think the practical approach to C# makes it a more useful language than Java, where Joy has some definite philosophical baggage he's carrying around that is reflected in Java.

Note we're talking a fairly fine gradation here. If you need to work in either, you're not slumming.