Why go through all that trouble? Those who are able to judge how difficult it is to answer a question, might well just answer the question.
And if I'm a user with not much experience, it's still not all that hard for me to judge whether or not I'm able to answer the question. Usually tags and title are a pretty decent indication. And if not, it will not take much more than a couple of seconds of looking at the question itself to see if I have a chance. (Of course I could spare myself that step since I'm a bloody idiot, but this is for the general case).
So you want to answer difficult questions? Well, if they are difficult, they might have been around for a while. Go and browse through your favourite tags. Go to the unanswered tab perhaps. Have a look at questions with a bounty. Those have been around without a satisfying answer for at least two days.
There are plenty of easy ways to determine the difficulty of a question, or to limit your search/filtering in such a way that you can find questions that might be of interest to you. Adding another layer of evaluation on top of this, in my view, does not add much, if anything at all.