"My Day" will offer you a clean slate every... day. You can then attach unfinished list items by tapping on a lightbulb in the corner. To-Do's "intelligent suggestions" will pull tasks from your old, existing and future lists, automagically prioritising those it deems most important.
To-Do will also include flexible reminders that you can bake in to recur on weekdays, Monday mornings, whenever you need 'em. While Wunderlist won't be getting any new features and will eventually be sunsetted later this year, the team plans to incorporate users' suggestions and feedback before Microsoft switches to To-Do. (It's already built an importer tool to move your tasks across from Wunderlist and Todoist.)
It's not a complete substitute for Wunderlist just yet: sharing lists is a feature that's apparently in the works, and the app will also eventually land in Mac, iPad and Android tablet editions too. For now, you can play on the mostly phone-based preview starting today.