In the beginning of the game, the one an only thing that mattered was the attack point of the monsters. High points were pretty much everything back then. After that, monster effects mattered most. Low attack points could mean nothing if they had a huge effect. Then, what became most important was the creature itself: if one of them was a Tuner, it could mean a very powerful monster summoned from the Extra Deck.
Today, the main concern of almost every deck are the monster's levels. Equal levels and easy ways to bring them to the field means easier XYZ summons. Some decks focus in a certain range of level and try to gather monsters in that range. But it doesn't mean all the effort has to come from one single part of the game. Let's review a card that was mentioned before: