I use the sublist feature often for related activities. I think it would be very useful if you could set the due dates for sub items based on the parent item's due date. For instance, I have a checklist for traveling. If I set the due date for the day I leave, I could have an item "check in for flight" one day before, and another for "notify credit card company" a week before, etc. I looked around but didn't see anyway to do that.

Additionally, it would be nice to be able to set the default due date separately from parent items. I use today's date for main items, but usually override sub items to "never".