Nice point to discuss....However if we think about Q2A default structure than it has expert role. Which I believe some one only can reach when they have some capability to giving answer more rather than asking questions.
I admit with having some badge or different class but I would prefer to assign it with the user role. So that's how also we can differentiate users with class.
Super Admin has class "qa-super-admin-user"
Expert has class "qa-expert-user"
and so forth
Because practically it is very hard to say that the user who is giving more answer he/she will never ask a question. So I would prefer to assign class based on role.
Also badge user will earn as per them questions or answer so that also will make not much difference. Even Q2A system allows set points and permission based on points so many things we can control with all these.
This is my thoughts but I also would love to hear what other say..