In case someone hasn't figured it out, if you sync your tasks with Outlook, you can use Conditional formatting to mark the tasks with incomplete subtasks so that they were easy to identify in the task list.
In Outlook, go to "View" -> "View Settings" -> "Conditional formatting" ->"Add".
Add a new condition (e.g. named "Subtasks") with the following rules:
"Search for the words" [ ]
"In": Subject and notes field.
Assign a special format. I use underlined text. Then all the tasks with incomplete subtasks are underlined in Outlook task list in that view.