Modern German didn't undergo the Norman Conquest, a mass influx of West African slaves, or an Empire on which the Sun never set, so it is much more conservative. The incredible thing about the Norman Conquest, linguistically speaking, is that English survived at all.
The great vowel shift happened in the 16th century and is responsible for most of these changes. The original grammatical simplification (loss of cases etc.) between 10-1300 is difficult to ascribe, as similar happened in continental Scandinavian languages (and the Swedes had their own vowel dance!) But the shift in words themselves came much after (and before empire).