My humble opinion as a newcomer - feedback is definitely the biggest attraction. It's depressing to see 6,000+ readers of my first story and only two actually rated it. I know from personal experience that most stories I download I get maybe a paragraph in and dump them unread because the subject matter or writing quality just doens't grab me.

I don't see any way you could force people to leave ratings since most downloads are probably by people who aren't members. I tend to download, copy into word and reformat the stories before reading them, by which time I've usually closed the browser - rating a story means going back and re-finding it. Anything I can think of that would compel ratings would also force most visitors away.

Perhaps some sort of incentive could be offered to those who rate a lot of stories...?