The ability to snooze a reminder in the iOS app when it reaches reminder alert is critical functionality. Ability to snooze by 10 mins, 1 hour, 1 day and maybe a custom time (e.g. select snooze period) really is crtical to make the iOS app userable.
Basic task creation into Wunderlist Inbox would be foundation functionality for initial release.
Ability to add due date and select specific lists would be a nice to have (for initial release).
Siri integration is an essential feature.