Apart from static, functional modules versus objects, you can attempt to get some of the benefits of F# by using C# 3 and lambdas, LINQ, etc. However, that doesn't go very far. What I find nice in F# is:
- Inference everywhere
- Auto-generalization (adds in type parameters so I don't have to sort it out manually)
- Easy immutability
- Easy mix between module and classes
- Types like discriminated unions
- Pattern matching
- Nested functions (lightweight)
- First class functions (no, C#'s named delegates don't count)
- Everything's an expression
- Easy function composition
So, you can try to do some of this in C#. Some of it is simply not supported and won't work. The rest gets very ugly, very fast.
So, if you go much off the beaten path of LINQ and the Enumerable extensions, you'll probably end up in a world of pain.