RememberTheMilk lists are organized by tabs. Items are easily entered (although there is an extra click in there that bugs me). Clicking between lists is very straightforward. Items can be easily reordered. And you can also share lists and/or choose to make them public.
One thing RememberTheMilk does very well is to allow lots of metadata to be associated with a single task. Priorities can be set with a nice color-coded system, and there is flexibility in setting done-by dates. You can also add notes to a task.
A really nice feature is the ability to add tasks via email.
Reminders can be sent via email, instant messaging or sms. You can also subscribe to lists via RSS.
Overall, using RememberTheMilk is a much richer experience than Ta-Da Lists. Setting date reminders is particularly useful. However, there is a definite tradeoff in ease-of-use. Using Ta-Da Lists require no training, while I seem to be referring to the RememberTheMilk FAQs constantly to understand functionality.