We often have some high-level project tasks with a few sub-tasks. Once the project gets big enough, we move the subtasks into a separate list.

This is painful with the current UI, because you have to either promote all the subtasks (and risk losing them in busy lists), or move the parent tasks, then promote all subtasks, and then move the parent back.

A one-step move-and-promote UI would be very useful.

This would also prevent users "moving" sub-tasks and actually accidentally moving the parent task.