ABBA's performance with the song "Waterloo" violated many of the traditional framework of the Eurovision Song Contest. Rhythmic, close to rock music, the song broke the familiar atmosphere created by the melodramatic ballads that made up most of the Eurovision repertoire. The band also impressed everyone with their outfits and some elements of the choreography.
Breaking the tradition was the fact that ABBA performed "Waterloo" in English. This was strategically important for the success of the group — Eurovision-74 was held in the English resort town of Brighton. The rules of the competition at that time required all participants to sing in the language of their country. An exception was made for ABBa, which was subsequently applied more and more often, until this rule was completely abolished. If you listen to ABBA's later songs, you'll see how their English has improved — it was pretty weak in 1974, and there's a noticeable Swedish accent in "Waterloo."
However, none of this would p