You should give me a better reason to sign up than just saying it's a minimalistic to-do app. A screenshot, a tour, anything.
Oh, I see you have a link to a nice screenshot buried under "Help". If you were to bring this link out into an introductory sentence before the sign-up form, it's look much better.
Another thing is requiring both email and username. You didn't build a social site, I don't need a username/nickname for anything. Usernames are annoying, just another detail to remember. Just use email as ID; if you want to go an extra mile, allow any id suggesting email address as the reasonable choice - look at instapaper.com for inspiration.
Oh, I see you have a link to a nice screenshot buried under "Help". If you were to bring this link out into an introductory sentence before the sign-up form, it's look much better.