I think FRP is a good idea; but it seems to expose a failing of many languages: the lack of metaprogrammability. In particular, most (non-Lisp) languages don't let you mess with control flow much.

The result is that when writing FRP code, users have to manage two control flows: the 'logical' control flow, which is expressed via whatever FRP library they're using, and the actual control flow, which is the one provided by the language they're in. Languages would need good support for source-to-source transformation, or, alternatively, allow the compiler IR to be manipulated by libraries.

From the little I've played with Haskell, it appears the monadic binding operator (>>=) lets you do this, and the user writes code in an idiomatic, imperative-like manner. It is extremely impressive that this capability can be used to arbitrarily extend the language, while being a very small concept.

Edit: not sure if I had my terms right on the operator.