I was told once by a history teacher that 1066 was the most important turning point of world history. Whether I believe that or not, I don't know, but it certainly was important. It affected the fate of the British isles which, as Britain and England would be so key in global history, in turn affected the world.
I know precious little about the affair, frankly. I know that there was a dispute over the throne of England and who was named rightful heir after the King had died. Harold assumed the throne. However, in the Normand lands in Northern France, William the Bastard also claimed the throne. The Normans thus invaded, took the English throne, and I believe used the British isle as an unimportant Norman holding they could tap money from for a long while. There were French Kings, French forced into use as the language of government, and a historic census somewhere along the line.
So what if the Normans had been defeated at Hastings and Harold won the day?