Things I would change:
(1) The top navigation at the first sight looked like some problem with your markup - <home You may want to change it to something prettier.
(2) There should be some way to directly navigate to other lists from the the list detail page.
(3) The design looks good, but I would play around with the colours a bit more. (Too gray-ish now)
edit: (4) Ability for adding sub-tasks is important.
I change < to an arrow image. Thanks for pointing it out. :)
I haven't had a need to jump from one list to another in one click, but I'll definitely keep that in mind in case it becomes a problem.
I debated with myself about the colours as well. I went through several iterations, eventually I decided to focus on the tasks, while keeping the rest of the app as less distracting as possible. As a result the text for the tasks are a bit sharper and darker than the rest. The idea is to make the frequent things clear and visible, while the rarer elements hide into background.