1) This is impossible. What exactly is impossible? Getting Digg and Reddit to adopt it? Maybe, but there are a variety of uses that I can think of that have nothing to do with digg reddit or HN.
2) This has been tried before. So what?
3) Twitter and Facebook do this already or something similar. So what? First of all, both facebook and twitter have totally different business models and really have no incentive to include reputation. Further, the mere fact that there exists a large competitor in the market doesn't mean a small venture that may be more simple, targeted, and agile cannot succeed.
Maybe I am being naive, and if so, I fully expect to hear about it. However, I find your arguments unconvincing, and I would be greatful to hear other people's thoughts on them.
Thanks again for the comment.