I actually have the same need - it'd be nice if there was an ID number field in the task so that I could collaborate better with other people and identify tasks more easily. Tags would not realy work - you can't depend on humans or clients to create unique sequential numbers.
I know that internally there is an ID - I believe the API lets you see it. It'd be nice if the user had the option of displaying it.
I was wondering if anyone as expressed interest in a higher level of sharing that makes the shared folder/items appear together with your own items? While I wouldn't want to do this for all collaborators, for some it would be very helpful.
It'd also be neat if there was a function while viewing a shared to-do list to copy an item to my own to-do list. Unless there is already some other way to accomplis this? I basically want to take the item from someone else's list and make it my own action item.