This is exceptionally well designed and thought through. It seems like it could be a lot more valuable than just Q&A for HN users, though that's a smart place to start. I'm impressed with the attention you've paid to some of the details, like the "Curious" counter.

One thing that has held me back from Quora (apart from needing no new ways to spend time online) is that absolutely everything seems public, such as who upvoted what. That makes me uncomfortable, like I'm under a harsh spotlight all the time. Most of the things I think, or say in conversation, are tentative and exploratory. I don't want to be publicly committed to them by default; I want to make that choice myself and be aware that I'm making it, like posting on HN. I'd be surprised if lots of users didn't feel this way, so maybe that's an opportunity for you to differentiate yourselves. In any case, I'd be interested to hear what you've made public and private by default and what your ideas are on the subject more generally.

It will be interesting to see what this grows into!