Comparison of C Sharp and Java

Similarities

Both languages were designed to be object oriented from the ground up.
Both languages rely on a virtual machine, in the case of C# the .NET framework.
Both languages have syntax rules which catch a large number of common programming errors (related to initialization, etc.).
Both languages include synchronization.

Tied more closely to the Windows operating system, making for better performance in OS-specific tasks such as user interface.
The .NET framework already supports a much larger number of languages, making for better cross-language compatibility.
Allows the use of pointers in a relatively safe way, and certain language features ensure this functionality is not misused accidentally.
Better support for arithmetic by including more primitive types and functionality to catch arithmetic exceptions.
Includes a large number of notational conveniences over Java, many of which, such as operator overloading and user-defined casts, are already familiar to the large community of C++ programmers.
Includes an enhanced for loop. (Java will include this feature in version 1.5)
C# is defined by an ECMA standard, whereas Java is proprietary.

Already implemented on a larger number of platforms, making for better cross-platform compatibility.
More mature, meaning a lot of problems have already been worked out, and more code is already widely available.
Language is simpler, making compile-time optimizations easier.
Method invocation model is simpler, making it easier to implement and at times easier to read.

Some would argue that C# is under too much control by Microsoft, making it a poor choice in the long run.
C# takes longer to learn.

Does not include operator overloading, making it ill-suited for scientific programming.
The lack of pass-by-reference for primitive types makes certain procedural programming tasks awkward.

C# does not include checked exceptions. Some would argue that checked exceptions are very helpful for good programming practice. Others argue that they were to some extent an experiment in Java and that they haven't been shown to be worthwhile.
C#'s namespaces are more similar to those in C++. Unlike Java, the namespace does not specify the location of the source file.
C# includes delegates, whereas Java does not. Some argue that delegates are extremely useful, but others point out that their inclusion complicates the method invocation model.