Here's an idea that should have been put into Limewire ages ago. Gnucleus had a feature which allowed you to automatically kill slow uploads and downloads. If the speed of a transfer dropped below a certain user-defined threshold value (let's say 1.0KB/s) for more than three sampling periods, the transfer would be automatically aborted. The reason for this is because you often see connections sitting there with an avg transfer speed of 0KB/s -- this is likely a long-ago dropped connection which hasn't been recognized by Limewire yet.

Another idea is to put maximum time limits on transfers. Again make them user-configurable. So this could be done by estimating an average transfer speed over a number of polling periods, and if the size of the file cannot be transfer under the time limit, the user is kicked off. This would, for example, prevent dial-up users from trying to do impractical superheroic downloads.