I think every programmer made their own task manager at least once in their life.. welcome aboard :)
Also, don't be to hard on yourself, creating a real project from scratch takes time and passion. I understood in half a second what your app was about and why I should use it.. What many apps/site cannot even do in weeks :)
Finally, I like the idea about adding/removing stuff by replying by email.. so intuitive but I never thought of it. Maybe a kind of "delayer" might be useful:
- Do that tomorrow night
- Do this next week
So at least, if your 'digest' is empty, you don't get an email but you're sure that you will receive it when it matters.