Actually, how do people buy stuff in Germany? I'm in the Netherlands and everything uses iDeal, which was annoying at first but now I like it. I use a credit card otherwise. When I was in Ireland I used a debit card which functioned like a credit card.
The only downside is that it's only for Dutch consumers, and therefore international support is very spotty (Steam supports it, Lego does not). This weekend I heard a rumour that they might be making it international, though. That would be really nice.
The biggest problem to making it work for all banks is that at the moment, you need to select your bank from a list. That list is going to get enormous. The browser should probably remember your bank for you.
I think the big difference in Germany is that people here tend to have their bank account linked with PayPal, not their credit card (because many people in Germany don't even have one).
Compare that to entering your "up to 34 alphanumeric characters" IBAN bank account number manually while on your phone.
Most of the options I observed are either PayPal, direct debit, Giropay, collect on delivery or credit card.
I absolutely prefer direct debit. Just enter your name and IBAN and you are done. No 3rd party websites, no fuzz.
It would be great if there was a popular app/feature so you could save your IBAN in your phone/browser and just fill it in with one tap. That would skyrocket adoption and let merchants move away from expensive payment service providers.