As for your code style: I don't really see the point of making AMOUNT_HIGHEST and AMOUNT_HIGH global variables, you could just as easily pass them into set_priorities (in fact if you did this you wouldn't need the temporary variable "counter").
You mean like arguments? Good idea, but I kind of need a clue on how I'm going to avoid the use of "counter" here... I'm quite sure I could do without, I'm just not seeing how really.
EDIT: using enumerate for that loop now.
I'm putting down some ideas of my own for the actual plugin:
You can have it auto-manage all still-downloading torrents, but you can also turn this off and individually manage the torrents. When auto-managed the plugin could check (or just set) priorities every one or two minutes.
When not priority-managed, right-click on a torrent and you can "prioritize its files", which sets the files to highest/high/normal priority, based on the two setting boxes in the plugin's settings pane.
The config panel could have a number input box for the minimum treshhold to enable the usage of auto-managing a torrent - for example, it's useless to set the priority to highest on a torrent that contains only one (or maybe two) files.
Maybe some compatibility with the label plugin, where only torrents with a specific label will be checked and priority-managed?