Judging a battle by its results
It is easy to say that 1066 changed everything. A more careful way to judge the significance of Hastings is to ask which long-term changes plainly followed from the Norman victory and which were already underway. Historians still debate the balance, and the answer differs by topic.
A new ruling elite
The clearest consequence was a change of people at the top. The English earls and many leading thegns died in 1066 or lost their land in the years after, and the king gave estates to Normans, Bretons and other followers. By the time of the Domesday survey in 1086, only a small number of English landholders of high rank are recorded as holding directly from the king.
This was more than a change of names. The new lords often held land in Normandy as well as England, so many of the leading families had interests on both sides of the Channel. For generations their attention was divided, and a good deal of English politics through the twelfth century revolved around this cross-Channel nobility and the question of who would rule both lands.
The church under new leadership
The Norman kings also changed the church. Archbishop Stigand of Canterbury, whose position was questioned by the papacy, was removed, and Lanfranc, a scholar from Normandy, took his place in 1070. Over time English bishoprics and abbeys were taken over by continental clergy. Church courts were increasingly separated from secular ones, and ties with Rome grew closer.
Another effect of this was a building boom. Large new stone cathedrals and abbeys in the Romanesque style were begun in many places, often replacing older English buildings. They show the wealth and confidence of the new rulers, as well as their desire to make their power visible.
Government and the crown
William and his successors used and extended the existing English system, which was already fairly developed by the standards of the time, with shires and local courts and a royal habit of issuing written orders. The English crown under the Norman kings added a strong emphasis on the idea that all land was ultimately held from the crown. Royal oversight, taxes and records grew. Domesday Book is the most famous example, but it is only one sign of a monarchy that kept a close eye on its resources, in a pattern echoed in later royal administration.
Later kings of the twelfth century built a more organized system of royal courts and financial administration on this base. How much of that came directly from the conquest, and how much from the energy of later rulers such as Henry II, is something historians disagree about, so it is safest to say that 1066 created conditions that helped it along.
Language and law
The effect on language was slow but deep. French became the tongue of the court and the elite, and English words and spelling gradually shifted as many French terms entered the vocabulary. English did not disappear, and it re-emerged as a language of high status by the late fourteenth century. The conquest therefore helped to shape the language we now call Middle English, but it did so across centuries.
In law, English customs persisted in many local courts, even as Norman ideas about land tenure and royal justice were laid on top. The result was a mix that eventually developed into what is called the common law.
England and the wider world
The conquest tied England to France and Europe more closely. Kings who were also dukes of Normandy, and later rulers of lands further south, fought many wars over their continental possessions. This is one background to the Hundred Years War, the long struggle between England and France in the later Middle Ages. In that sense, a battle in Sussex helped to set the stage for conflicts that lasted centuries.
What did not change
Daily life for most people changed less than the names of their lords. They still plowed, sowed and harvested within the manor and village routines, and many of the local institutions of English life kept going. The Norman Conquest was brutal for those who lost land or lives, but its biggest effects were felt through the top of society and through the structures of government and church. Even those are best described as a mixture of change and continuity rather than a clean break.
Why the debate continues
Because the sources are limited and often written by the winners or by clerics with their own agendas, historians reach different conclusions about the speed and scale of the changes. For readers, the safest summary is that Hastings did not alone create the medieval English state, but it determined who would shape it and which connections it would have for a very long time.