I don't really care for Ed Helms style of comedy. He often comes across as being too pretentious and smug about how intelligent he thinks he actually is, as opposed to his characters just being pretentious and smug asshats.
I also don't really get a Rusty vibe from him at all. Say what you will about the character, but he was actually pretty smart and charismatic, so following in his dad's footsteps as a bumbling, oblivious father which I'm guessing he'll become since they are actually going with Ed Helms, just doesn't seem to fit. None of his incarnations had dark hair either, so there's that small detail, too.