Mass Diminish

Dragons, alongside Goblins, are reds primary creature types. Dragons are reds strongest creatures and most of the expansions have some kind of red dragon in them. As dragons are expensive to cast, dragon tribal decks run multiple cost reduction effects and have ways to cheat the expensive dragons on the battlefield.

Mono blue has multiple ways to clone own or opponents permanents. The gameplay changes effectively, when you get to play your opponents biggest threats, too and clone decks can lead to some very weird and interesting situations.