I would say the battle of Hastings is one of the most important battles in medieval western European history. The Battle of Hastings ended centuries of Anglo-Saxon rule in England and caused the start of Norman rule. William the Conqueror and the Normans kicked many of the Saxons off their estates and replaced them with Normans and other supporters of William. About 20 years after the conquest not a single Earldom was held by a Anglo-Saxon. The same happened in the churches.

Many Saxons fled England, Harold Godwinson's descendants tried multiple times to retake England. A lot of Saxons fled all the way to the Byzantine Empire and many that settled there joined the Varangian Guard and even replaced the Scandinavians as the main group of people in the Varangian Guard.

Slavery was very common in England before the Saxons, but after the Norman Conquest slavery in England became less and less common.

The English language would not be what it is today with out the Norman Conquest. Many french words and names entered the English language, William and many Normans used what would be called "Anglo-Norman French". The language impact can still be felt today, even though English is a Germanic language a good chunk of words in the English language come from French and in particular the dialect used by the Normans.

William was the Duke of Normandy so he was a vassal of the King of France before the conquest and for many centuries after, Normandy would remain as part of the Kingdom of England.

Here is a small fun fact: All the current European monarchs today are descendant of William the Conqueror (and so are many other people of Europe I'm guessing).

There is so much that Norman conquest caused that I don't think I would have enough time to talk about.