Another problem is that as nice as avoiding state and side effects is, for any decently complex application there's a minimum amount of state that you simply have to handle, and as a general rule OOP languages seem to make dealing with state much easier. Mind you there are plenty of good ideas in FP, and discriminated unions is the feature I want added most to C#.
> as a general rule OOP languages seem to make dealing with state much easier
I disagree. One of the most significant benefits of FP (and pure FP in particular) is being able to deal with mutable state safely in concurrent programs. This is essentially the core problem which effect systems try to solve.
