I find TD almost perfect as a to do list and note taking tool, but I have to agree the UI looks dated. Look at AwesomeNote or Wunderlist. Just the relatively simple ability to change up the way it looks day in and day out would be a welcome addition.
You can always keep the simple, minimal look as the "classic" view, but allow color, layout and style to enhance the 'usability' a bit.
I would also suggest having high priority items a different display color, or allow grouping of tasks by color (even though this would be less useful for me personally). There is a reason infographics have become so popular. You can convey a lot of information graphically that is immediately understandable.
I think that this visually pleasing element is what keeps TD from being the best (or certainly one of the best) productivity tools on the market. It just looks a bit dated...