1) Follow system like quora will be better. I think the point system is not a good idea. People give a short answer for points. In a follow system, people will give a big and better answer to gain followers.
Users can follow different topics or categories.
2) On the home page, people will see questions from their followed users and categories. if I post any question or answer my follower will see this on their homepage. Everyone can see questions and other pages without registration but to get access to the homepage they have to register.
3) Users can request other users to answer their questions. if I ask a math-related question I can see those who answered in the math category before and I can request them. They will get a notification and email.