In a nutshell, the Roman Empire was destroyed by barbarians because of qualities of the barbarians, not because of any incipient weakness or any other quality inherent to the empire. I can't judge how original this is for people familiar with the field, but for me it was a completely novel way of looking at a central event in world history, that I didn't even know was open to question.

Maybe this deserves 5 stars... I didn't appreciate the massive detail of political intrigues and economic developments over 2 centuries perhaps as much as I should have, or the work that goes into making such erudition even comprehensible to a non-expert.