I don't feel so good about bumping people into the queue, even though I can see from your example how it would theoretically work nicely.

For one thing, connections seem too unstable. It might become very difficult for rare files to propagate if a steady connection were not guaranteed.

The additional problem is that the original downloader might never regain the connection if they can't make it into the queue. I share a lot of files, and I always have far more people trying to connect than I have slots.

I like the idea about only letting so many slots go to a single file. When I have a popular file in my shared folder, my uploads are dominated by people trying to get that file. Something to think about for future development might be something like bittorrent support, like Shareaza has, so that popular files can be shared more effectively.