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Part as in idea in the series or as in actual plot?
If as plot: the Quidditch World Cup (and I don't mean the whole Deatheaters stuff afterwards). The Quidditch World Cup is the first time where the rest of the world is referenced in the series and how there other countries with their respective magical authorities, etc
If as topics: how it introduces in a easy way concepts that have a very heavy impact and that are important that people recognize like governmets lie, politics are dirty, abuse of power, homophobia and discrimation, war, death, etc0