Stackoverflow and all its sisters are very frustrating to use. You have to earn the right to do anything except posting questions and answers. Yesterday I was discussing this with a colleague, Chris and he enlightened me why.
I was unhappy that I had to earn the right to add comments in order to ask follow up questions. Given a specific question and its answers (of which one is the accepted answer), I might have questions about the answers themselves. Or the answers could apply to the person who posed the question, but not to my situation. It is hard to provide an accurate and universal answer especially for any Engineering related question as Engineering problems tend to be complex. I could have questions about the question itself too. Some people tend not to be specific. Chris said that Stackoverflow discourages discussions. They want high quality questions and answers. An answer is considered high quality if it is accepted by the person who asked the question. Upvotes also help increase its credibility. Another reason to make people earn the right to comment is to avoid silly comments like '+1' and 'Thanks'. And to ensure that people are being fair while upvoting/downvoting questions and/or answers, the right to do so also has to be earned. You earn these privileges by increasing your reputation. And how do you increase your reputation? Simple, answer as many questions as possible and ask good questions. Your reputation increases with the upvotes you get for your questions and answers and decreases if you get any downvotes. The downside is the competition. Everyone wants a high reputation. So people answer questions aggressively. Chris said that he's frustrated every time he tries to answer a question and someone else answers that question just seconds before his answer. If the other answer gets accepted, then it's a waste of effort to have answered the question. The accepted answer tends to get upvotes.
Chris said that despite all of this, it is still possible to have discussions on a topic. Stackoverflow and all its Stackexchange sister sites have meta communities just meant for discussions. That is exciting news for me because I want to be free to ask as many questions as necessary to understand concepts. He said that another way to deal with follow up questions is to ask another question and describe it clearly enough to distinguish it from the initial question. He explained that this also improves the Search Engine Optimization for that particular topic as users would get all 20 questions related to it and any one of them could apply to their situation. Comparatively, it's harder to find things if related questions are hidden in follow up comments to a question.
After a long discussion with Chris, I understood Stackoverflow much better. They want to have responsible users - people who contribute by answering other people's questions, not just by asking questions. Their vision is a high-quality reference on every topic created and maintained by the same people who use it.