Well it would seem that the logical next step is going to be giving it a way to provide permanence to the data. That generally means online storage, but there might be other ways that are more in keeping with your offline approach... or maybe you could give the user an easy to remember token that they can use to fetch their todo list from another computer. I like the shopping list aspect of it. I would focus my energy completely on the usability side. Keep optimizing for convenience.

I've just launched a todo list sort of an app too, but it borrows heavily from open source components, much hairier, different audience to yours I suspect (alouy.com).