First thanks to Przemolog, who edited my first post on this page with the updated comparsion table now containing minutel's "plus" tariff.
@Motel75
I think minutel internationally don't offer free forwarding to North American numbers, since on the one hand overseas roaming still delivers high margins compared to European countries and on the other hand free diversions to North America could have an especially strong impact, as this would include forwarding to American mobile phones (there are no dedicated area codes for mobile phones in North America) and so evading roaming tariffs would be very comfortable and cheap. As North American phone networks use the bill-and-keep system, which means there's no termination fee for calls going to another operator's network, there is at least no financial reason for the lack of free forwarding to North America. Forwarding to Eastern European countries is way more expensive due to the high termination fees there.
@Evan
This article from teltarif.de mentions minutel being "quasi-prepaid". That means the tariff is based on a postpaid billing-platform, which may not run always in realtime resulting in the theoretical possibility of generating a negative balance. That's probably why minutel's terms and conditions contain the right to check customers' creditworthiness with Schufa (a German credit investigation company). So I doubt non-residents without Schufa-record can order minutel.