Knowing C# and .NET
I believe that it's important to know more than the bare necessities about the environment you use when developing.
Not only might a lack of knowledge leave you at a disadvantage when reading or maintaining code that has been written by others, it can leave you working harder than necessary.
Why write 10 lines of code to achieve a result that can be written in one line? If you only know the bare essentials of the language and associated libraries, you might be doing just this.
In this series, you'll find blog posts detailing useful things about the C# language itself and the standard libraries that come with .NET.