Coming from using D for personal projects and now working in C# at work (in .NET 2.0 as well because of Unity) I find myself constantly missing D features. Especially given one of the main requirements of the work I'm doing is performance.

Things like array slices being first class in D while ArraySegment<> looks like a hack in C#, D templates beating the hell out of C# generics, functional programming in D is actually inlined almost 100% at compile-time while LINQ adds noticeable overhead, and the list goes on :)