It's a little too easy to add custom fields to the List level without realizing that it won't be available to the Folder. (When you edit fields while inside a task)
Could you make it possible to move it to the Folder level or vice versa? Or at least the ability to copy a List field to Folder field level?
Worst case, maybe something telling you that you're only adding it at the list level? I spent a chunk of time creating some fields, and just now realized they are only in one list.