It seems that also Mastercard debit cards are not accepted by some stores in Germany, like Saturn, even is Mastercard and Maestro are from the same company.
Anyway, it seems that Mastercard debit cards (embossed and with chip) are accepted in more stores than Visa Electron debit cards, which are not accepted by almost any store in Germany...

It seems that this is something particular to Germany - all debit cards and credit cards, from any country, are accepted by 95% of all stores even in countries like Romania, but I also used them successfully in Netherlands, Italy or Denmark without problem.