Is there a way to automatically get mobile AND email reminders for a task by default? I can only find a way to set one or the other.
Unfortuantely, not. At least not by default, you can of course add multiple reminders for multiple methods and times to each task manually.
We will consider adding default options for that in the future and you're welcome to request this feature in our Votebox: http://todoist.com/Vote/