We use stories (with story points) to manage our sprints, and sub-tasks for the actual to-dos within a sprint (without any estimation). When we configure the agile board to use "Issue Count" for "Estimation", the burndown chart shows the stories completed. It would be nice to (maybe optionally) see the much more fine granular sub-tasks burning down.

I would have thought this was a quite common practice, but I haven't found any similar requests. Some colleagues misuse the time tracking instead, by entering the number of open subtasks as estimated days.