Generally speaking, the queue becomes rather difficult to use when there are more than a few torrents in it.

For instance, there's no way to know - from the queue alone - whether a torrent is currently leeching or seeding. I assume that the downloading and uploading queues were merged in ktorrent 3 in order to facilitate prioritizing what's uploading (since both the leeching and seeding torrents upload), but it makes it a lot harder to prioritize the downloads. Maybe it would make sense to have a separate download and upload queue with only the leeching torrents in the downloading queue and all torrents in the uploading queue (since they're all potentially uploading). That could even make it so that a leeching torrent could be prioritized to download first but upload last - or vice versa (though presumably - unless you want a leeching torrent to be able to upload but not download or vice versa - whether a leeching torrent is running would be based on the download queue). But assuming that you don't want to make that kind of change, perhaps the status column could be changed to be more informative than "Running" and say whether a torrent is currently leeching or seeding - or a separate column could be added to indicate that. Personally, I rarely care about what's where in the queue with regards to uploads but I definitely care about the downloads and it can be difficult to find them among all the torrents I have running. Even just marking them as downloading or uploading would help a lot.

Also, there is no connection of any kind between the main window and the queue. It would be of some value if there were. In particular, it would be nice if selecting a torrent in the main window selected that torrent in the queue if the queue is currently the open tab on the bottom (similar to how the info tabs change what they display based on the currently selected torrent). The focus doesn't need to change (since you can have torrents highlighted in the main window at the same time as the queue), but having the torrent highlighted in the queue would make it easier to find (naturally, the queue window should scroll to it as well, since it wouldn't as useful if the torrent were highlighted but hidden). You might also make it so that selecting a torrent in the queue selects it in the main window (assuming that the current tab is for a group which contains that torrent), but that's not as critical.

It would also be nice if there were some way to separate the queue from the main window. I find it much harder to deal with as a tab since I don't normally run ktorrent maximized but I pretty much have to maximize it and resize the queue window in order to get it big enough to do much when there are more than a few torrents in the queue. Now, that may be too much of a UI change, and maybe there's a better way to fix it, but at present, the queue is in a place where it's difficult to use without resizing it while using it.