Key to what people appear to like is how the app stresses the social aspect of calendars, namely that the chances are good that if an event is sufficiently noteworthy for a user to warrant being recorded then it's probably also noteworthy for their friends.
But the description of how you connect to others is nothing new:
This model distinguishes 3D Boxes from, oh lets see, no other social applications. You provide your friend's email, they get invited to create an account, their new account is given whatever rights you assigned them etc. Viral indeed.
Central to 30 Boxes is the concept of buddies. Basically in order to add someone as a buddy on 30 Boxes all you need to do is enter in their email address (talk about genius viral marketing). They then are sent an email inviting them to 30 Boxes and if you choose you can allow them to see your calendar, see only events tagged a certain way in your calendar (cool, tagging in a calendar app), or see none of your calendar.
As to the calendar functionality itself, seems very similar to Planzo and others.