The text is difficult to read (which seems important to a text-based todo list).
Marking a todo as done is a two-motion action: hover over the todo, then click the checkmark (assuming you see the checkmark). The transition to the done section could be smoother as well. I didn't actually notice what I'd clicked for a moment - took a bit of analysis to figure out that I had marked one as complete.
I should be able to move a todo by dragging any part of it - not just the obscure drag handle represented by the menu icon off to the side. Note: I personally am aware that this is also the drag handle icon, but I know plenty of people that aren't.
I can make blank todos. Maybe someone, somewhere, would like to make blank todos - for the other 99.99% of your users, this is a bug.
The keyboard controls are unintuitive. Pressing the left arrow at the beginning of a todo takes me to the previous todo (bug?). Why would I want to indent a todo? "Make a subtask" sounds reasonable, but "indent a to-do" sounds like a useless thing to be able to do. Also, I can't move between the done and not done sections with the arrow keys.
I kinda expected (hoped?) that I could use Vim keybindings to get around. Nope. Not a huge deal, but you might want to look into that (or maybe offer different keyboard shortcut modes - Vim, Emacs, or Pie).
"Sign up to save your lists" sounds like something I'd want to do; however, when I click on it, I go to some other (nearly invisible) screen asking for an email and password. I had a nagging fear that my lists might not have been saved. It looked like the only way to leave the signup page was to click on the Home link, which took me back to the beginning. Then, my first though was "Shit, are my lists gone?". They weren't, to be fair, but that sort of panic-inducing moment is unacceptable.
The entire interface is almost invisible.
Overall, I applaud the effort behind trying to make todo lists simpler, and as much like keeping text files as possible (but with the convenience of being on the web!). I feel like the execution just really fell short.