The Q&A site has actually been an interest-based community for some time. Users have been able to follow both others on the network and threads on specific questions as updates in their news feeds. But there have been a couple of factors that limited its capacity as an interest-based network. The first is that there has been no good way to curate questions under topics that can be followed. Quora introduced "topic groups" in June to solve this problem by creating feeds of questions relevant to certain topics, but they aren't available for every topic and can't be created instantly.
The other factor holding Quora back from functioning as an awesome interest network was that the feed was restricted to content created on Quora, whereas other social networks like Facebook and Twitter have the wealth of the Web to draw upon.
Boards hit both of these birds with one stone by allowing users to create new topics with content from both Quora and elsewhere.