Unions remain the killer F# feature missing from C#.

Also, basic object initialization in C# has turned into a nightmare with recent versions. You need a flowchart to select among the 18 syntax options which suite your current needs.

With F# (and other newer languages), record fields are either `T` or `T option`. No need to worry about whether the value needs to be computed in a constructor and then remain immutable, whether it needs to be initialized by an object initializer and/or a constructor or not, whether it needs to remain interior-ly mutable throughout the life of the record, and so on. (Although as I recall you do still need to consider null values assigned to non-nullable references in your F# code that consumes C#.)