I love RTM, but I consider the web app UI as the worst one from the available ones (iPhone, Android, iPad). I think it should be reworked and maybe the iPad interface should be an inspiration for it :)

One must have is a horizontal list of user's lists. The current line-based is not very comfortable and does not encourage me to create a list (= project) for every small thing that's going on in my life. Quick list adding from the main screen would be also helpful for this.