Priority is essential to assign limited resources on competing tasks. Three levels of prioritiy are unable to manage a decent ranking. Providing a wide numer of priorities you could allow the users to manage easily this need and have an immediate feeling of what to do viewing the tasks sorted by priority. You could add the priority as as free input on the right pane allowing only integer numbers; you could add + and - links to increase or decrease priority. If you like to make us very, very happy you could show in a pane all the tasks having the same priority of the current priority (the selected task). Please, keep in the combo box in the main pane the options to increase or decrease the priority for all the tasks listed