Part of the problem a system like this would address is that, as far as reputation, there is less incentive to answer a difficult question that requires some research than there is to answer an easy (for the answerer) question. It can be frustrating that a more advanced question just gets lost, even if it gets upvoted, when you are the poster or someone who would really like the question answered. I know the "bounty" system is designed to address this somewhat, but because of the delay before a bounty can be offered, it is difficult to get a question answered in a shorter amount of time. (While I would be in favor of reducing or eliminating the bounty delay, that proposal has been rejected.)
The difficulty rating, I think, would need to be provided by the answerers/moderators, not the asker. I think it would be possible over time to "start" a question at a given difficulty rating based on the history of the asker's questions. Perhaps the difficulty could be expressed as a reputation "multiplier"; that is, a question rated "1.3" in difficulty would earn 1.3 times as much reputation for an accepted answer as a question with difficulty "1".
I'd really like to see an idea like this taken seriously so that more difficult questions can actually get answers in a timely manner.