A really great feature would be the ability to put one item in multiple lists, and have changes affect all copies of that item. To illustrate what I mean, in the example below, suppose that instead of having an item in 'list1' called 'linked_item' and a duplicate item in 'list2', both occurences of 'linked_item' are the same object. If I edit one of them, the changes are reflected in the other.

* list1
    * linked_item
    * foo
* list2
    * linked_item

But there's a glaring problem with this: sometimes I might want to have two unrelated items with the same name. In order to distinguish duplicate items from items that are "linked" like this, the text of linked items could begin with a special symbol. For purposes of illustration suppose that symbol is an exclamation point. In the following example, there are two distinct items named 'foo', but every occurence of '!buy groceries' refers to the same object.

* today
    * !buy groceries
    * !work on report
* category1
    * !buy groceries
    * foo
* category2
    * !work on report
    * foo
    * call joe

This example also illustrates a possible use case of this feature; namely that I can have a todo list broken down by category, and also a 'today' list. When I mark '!buy groceries' as complete in my 'today' list, it is simultaneously marked as complete in 'category1'. Likewise, if I mark '!work on report' as complete in 'category2', it will also be marked as complete in 'today'.