My problem with functional programming is refactoring. If you don't have side effects when you want to do 2 things deep into the call tree to something that is in completely different call tree branch - you have to extract it all the way up to the common parent and pass it through all the intermediates just so that one function deep there can access it.

It's incredibly frustrating when you work in a functional language, and yet it's the main benefit (no side effects).

I'd like to have a language that is imperative when written and functional when read :)