Something as simple as calling a .NET function that doesn't have F# bindings forces a change (e.g. `someLibraryFunc(arg1, arg2)` instead of `f arg1 arg2`).
This gets worse as libraries force you to instantiate objects just to call methods which could have been static, or use ref parameters, etc.
I say this as somebody who loves F# - you do absolutely have to know .NET style (which really means C# style) in addition to F# style. It's extremely pragmatic if you're already familiar with C#, but I'm not sure how wonderful it is if you come in cold.