Dell just relaunched their community for IT professionals, rebranding it from AppDeploy to ITNinja, and changing the interaction model from forum to Q&A. This follows the Q&A models used in other IT communities like Spiceworks and StackOverflow. It's a subtle but important change. A lot of forum discussion takes the form of Q&A anyway, but adding the formal structure of Q&A has advantages.
The core difference is this: a person asking a question is generally seeking answers, while a person making a comment is often seeking an audience. The person seeking an audience tends to have a more "selfish" set of motivations, like establishing a reputation or promoting an agenda.
Those goals are often better served through vehicles like blogs where the commenter has more control over how their contributions are presented, and where their posts can be easily seen in aggregate. Hybrid community/blogs where a newsfeed spans across posters but individuals' profiles are still prominent (like Facebook, Twitter, and Tumblr) satisfy the goals of commenters particularly well (thus the success of these platforms - there are a lot of commenters out there!).
On the other hand, commenters who participate in forums tend to be motivated more by the desire to be helpful than by the goal of self promotion, and if your goal is to be helpful, answering questions is a great way to do it! So, community forums tend to attract answer-seekers and answer-providers, making Q&A the natural framework.
Of course, the distinction isn't black-and-white, but Dell's decision to switch from a forum model to a Q&A model is further evidence that Q&A is gradually taking over from forums as the interaction model for communities.