Follow how http://growthhackers.com/ has done it / it's Q&A plus a resource for links and community. Pure brilliance.

IMO Quora started great but now they are trying to do too much.
So, I would just pick up Quora and would start to remove things. After you remove A LOT of stuff, remove more.
I would also try to create a single reply per question (kind of what RapGenius does for lyrics) needed time to think a way to make this work properly but I think there is something interesting that can be done there because most of the time you end up having to read a bunch of answers to get to a conclusion.
But essentially I would remove stuff and would start with a niche.

Before starting anything I would start answering the following questions and make sure the answers don't come from you (or an assumption) but from the niche:
1) Is there a need from the niche to have a Quora like service?
2) What value would this bring for the niche? (Think of what your users' objective is and how your service would help them)
3) How Quora lacks in serving this niche? (What are the main defaults of Quora for your niche) Perhaps to many question, not well organized,...

If you are building a Quora like, you should not copy Quora but emphasize your differences with Quora so it brings the full value to your niche.

You should also aim at building a core competence that will differenciate your service from Quora on the long term. What if Quora decides suddenly to build a similar service to yours? What will make your service better than Quora anyway?

Those answers will not come straight away but you should keep them in mind along your business design process.