Something interesting to try is “Remember the Milk.” It is similar to alot of calendar task tools, with some very interesting approaches to keeping you abreast of what needs to be done.
While it has nothing to do with the actual functioning of the site, I really like how the registration screen uses ajax to confirm that your data (eg. email) is valid while you are typing it in, and then gives you a checkmark when it is complete to satisfaction. That kind of detail to improve user experience is pretty symptomatic of what the rest of the service will be like.
I also see quite a bit of sharing — Google Maps for locations, iCalendar compatibility, Google Gears compatibility so you can continue to get your tasks offline, you can add tasks and receive notices via Twitter and lots more.
There are a wide range of features as well — tagging, shared tasks and so on. I think its a worthwhile product to try.