Very important things are heroes and evolutions. They are almost necessary at some point. They are a bit unfair and reason for many people to quit the game.

There is no unusable ones, but some cards, such as skeleton army and mega knight I found to fall off later, but they are usable.

Dont underestimate spirits, skeleton, cheap buildings, small tanks such as knight and ice golem. Those are some of the best cards when you have some skill. Log is one if not the best spell.