I'd like to present an idea. Downloads should be controlled by bandwidth, not quantity.

You tell the file-sharing program it has X KB/s of bandwidth to use. You then add a handful of downloads to your queue, and the program begins downloading the first one. After a period of time (30-60 seconds?) for the download rate to stabilize, the program checks the current download speed to the total bandwidth, and if there is sufficient bandwidth left, it will begin another download. Repeat until X KB/s is used.

Does that make sense?

Nice idea - and also something similar for uploads. If I've kicked of a large download from someone and I leave my computer switched on overnight, chances are that the upload will finish long before I next return to my machine. I'm a broadband internet user and so a fair bit of bandwith has been wasted. Why not free up all available bandwith for uploads while my machine is idle?

And as stated above, dynamic allocation of connections according to available bandwith is a v. good idea - in both directions.