If a person answers with a follow-up question, and the author of the original question replies with a comment - how will the answerer ever know?
If somebody answers my question, will it move to the top of the questions listing for the tags that the question and the answerer have in common?
Why is there no private message system on Stack Overflow?
Time and time again I find that my questions die out because of 1, 2 and 3. The answerer never gets to know her answer needs more answering—the question is too far down the list to serve as a reminder—and she didn't leave any contact details in her profile so it's impossible for me to contact her and offer her payment to complete the answer.