I haven't been able to find the following feature set all in one place:
offline access :: Something which guarantees offline changes aren't lost.
synchronized :: It should synchronize across my iPhone/iPad/laptop, either via iCal TODOs or whatever.
0 wait mobile entry :: Related: some mobile apps synchronize before I can enter in a new item. This sucks. I want to be able to add a TODO on the go with no wait (and definitely with no wifi).
no overdue, or autoadvancing overdue :: I don't want to spend any time maintaining my todo list. If something is overdue, it should autoadvance to my "today" view/inbox or whatever. I should be able to see at a glance all my tasks (including overdue ones) that I need to do today.
due dates optional :: I shouldn't need to assign something a date.
Bonus points for something that's stored as a simple text file. I'm open to solutions like Notesy + nvALT + Dropbox. That particular combination fails because I use Notesy for other notekeeping, and so I can't satisfy the zero-wait constraint.