Traditionally, blog themes have placed the form to leave a new comment below the listing of existing comments. This still holds true in most WordPress themes to date. However, many sites now have the comment form above the comments. Notable sites include Mashable and Reddit.
I was pondering the reasoning behind this recently. The obvious answer is that it encourages more comments, as someone who just finished the article is prompted to post while the content is still fresh in their mind. Meanwhile, having the form below the comments requires that a user read (or simply scroll past) others’ messages—perhaps even seeing like-minded comments and deciding against leaving one.
Assuming this is true, you could promote a higher volume of comments by placing the form higher up, or on a higher traffic site, promote a higher quality of discussion by putting it toward the end. I assume someone has done tests, but I couldn’t find any publicly-posted results.