QA is some kind of “by word of mouth”. It means some user will use QA website to build up their reputations in the industry. For example, if some one asked a question about repairing a car. Tom as a expert can help people to answer related questions to get himself well known to promote his own business.
If there are functions like Follow Tags or Topics You May Be Interested, It will save Tom time to target his potential customers. It will make Tom more willing to visite the QA site.
In designing a QA site, we should keep in mind that are two group of people are most likely to visit this kind of site.
One is those people who has emergent questions to ask. Or who has got a satisfying answer before and comes back again.
The other is those who are making profit or reputation by answering quesitons. They are the mostly helpful people to make the site active.
When designing, we should better match their needs respectively. Basically, efficiency is the most important thing. For questioners: Quicker the answer comes, stronger the will to visit again. For answerers, quicker the question comes, the stronger the will to use again.
Hope this idea is helpful.