What makes people respond to discussions?

This would depend as much on luck as it would on the quality of the topic. You will notice that new discussions are listed in a chronological order, so it does not take long for your post to disappear from the early pages. If someone responds then the discussion moves back up the pages, but if nobody responds for a while then it will sink down again. It does help to have good friends because they will notice the post in "discussions my friends started", and another good trick is to reply to your responses over a period of time. each time you reply the discussion will be seen again for a while.

Any general topic is good and you get good response. If it becomes very specific, chances are less. It also depends on how you present your matter.