Since I posted on this thread, i've had several opportunities to see the Assassin in play as per the Revised 4th Edition rules.
Actually, I think he holds his own against other characters but doesn't overpower them. The ability to assassinate "board space" enemies such as the Guardian and the Pit fiends didn't have a huge impact on the game and the Assassin still can't assassinate Craft creatures.
All in all, not bad. I lay my previous fears about the potential Assassin overpoweredness to rest.