As I said on another topic, I’ve just discovered Vikunja which I find very pleasant.
From my point of view, a little trick that could make the experience smoother would be to decrease the number of clicks and mouse movements needed to edit a task. For example, if I want to had a priority to a task which doesn’t have one, I’ve to click 3 times: on “set priority”, then on the field (after having to move the mouse cursor!), then on a priority. Perhaps a click on “set priority” could display the list of priorities at the same place?