Because of lifetime of captured variables in closures, mostly.

You can't really pass around functions freely like you would in Haskell/ML unless you start Box-ing/Rc-ing everything. But then it's not really idiomatic Rust anymore, IIUC.

Rust doesn't really have first-class functions - you can't take an arbitrary snippet of code and turn it into a function value, because sometimes it will no longer pass the borrow checker.

Well, one big one is lack of tail call elimination. If you can't write even a simple tail-recursive algorithm in the language, it's not a functional programming language.