Which posts attract the most comments?

Some posts attract many more comments than others. Holding constant the general level of readership, I believe the following features predict higher numbers of comments:

1) If a post sits at the top of a page for a long time

2) If the post invokes a sense of moral outrage

3) If many readers feel they can bring facts or personal experience to bear on the question

4) If the post simply asks for comments or feedback

What do you all think?