I didn't sign up for To-Do, I signed up for Wunderlist which I understand MS will be retiring in favour of To-Do?
That being the case, an absolutely must-have To-Do feature is the ability to share lists. My wife is a technophobe but she loves Wunderlist and the way we can share various Wunderlists! We have practically organised our lives around this app - from the shopping list to camping trips to various projects around the home and garden and more. Between us we have dozens of lists on the go!
Also are we safe to assume that by continuing to use Wunderlist, there will be a tool for importing "Wunderlists" into To-Do before the switchover? This is something that some competitive products (such as todoist) offer already
BTW Wunderlist is so useful to us, that so long as To-Do provides the same level of basic functionality (esp. shared lists and ability to prioritise items) we would be prepared to pay a reasonable subscription fee, as per some of the Wunderlist/To-Do competitors :)