Updated Feb, 1st 2013
Yes, the world has a lot of to-do apps already. But I haven't yet found one that works my way.
I'm not interested in the details of the "Getting Things Done" system, or spending hours creating an exacting architecture to keep track of my entire life. The things I need to worry about don't always have due dates that align nicely with a calendar. There are a lot of them, but they'll get done as I do them.
What I want is the same sort of thing I used to do on paper. I want to write down what I'd like to accomplish for the day. If I don't get it done, well, that's okay; I'll move it onto the list for another day. Maybe I'll get it done then; maybe I'll need to move it forward again. I don't want the iPhone to bark at me or warn me or complain; I just want it to keep track of my to-dos and let me organize them as I will.
So, this is intended to be an application supporting a number of separate to-do lists, with names like "Saturday the 2nd" and "Sunday the 3rd" (which could be auto-generated to a horizon specified by the user). Extra user-lists could appear at the bottom, for GIT-type ideas like a "Someday" list, or for reservoirs of items to move onto the day-to-day lists as opportunity allows.
Drag-and-drop prioritization is a must. (An app preference can specify whether, when you send an item to a different list, it goes to the beginning or the end.). And items should have sub-lists, as with phone calls to make, emails to spend, or Internet research to do, so you can move all those tidbits en masse.
Spacer elements (to separate notional groups, like tasks to do on errands vs. at home) could be handy (and would probably consolidate when two collide). Color-coding of tasks would also be a good option (so you can use blue for personal tasks and orange for household tasks, or code by important or urgency, or whatever).
The screen's footer ought to have a small pencil icon. You can click on it to add a new item to the top or bottom of the current list (as per the app's preferences). Or you can lay a finger on the pencil and then drag (with a blue line following) to add a new item in the appointed spot. Another icon next to the pencil should perform the same function for a neutral spacer.
Clicking on a specific item should allow access to some options, such as editing its text, moving it to another list, adding sub-items, changing the color, etc.
I'm hoping this is simple enough for a beginning project, yet complex enough to be useful. I look forward to working on it!