I have had the pleasure to program in a variety of programming languages throughout the years including the trifecta of C++, Java, and C#. It's often interesting how these three languages are so similar and yet have such key differences as well. Each of them has features that I miss when I work with the others. But, if I had to pick one standout feature that I love in C# (and long for when working with the other languages) it would be LINQ.
There is a wealth of algorithmic and framework components in LINQ that make development more rapid and code easier to maintain. Extension methods, are a ubiquitous feature of LINQ even if you don't know you are using them. And even without using LINQ, they are a very interesting feature of .NET in of itself and worth a more detailed discussion.