After many requests we have now added the ability to drag a task from one list across into another.
Simply move your mouse cursor over the task you wish to move. Click and hold on the up and down arrow at the front of the task and then drag this task across to the new task list you want it to be a part of. When you are happy with where the task is just release the mouse button.
You also have the option as well to drag tasks from a list and drop them directly onto a task list name on the left-hand side as well. This works really well if you have a lot of task lists as it saves you scrolling up and down the page.