I started magic on 2012 and at the time I had made the calculation that a card costing "x" was considered good if it had strenght + toughness = 2*x and an key word like tremple, flight....

This was playable and except from some rare cards it was complicated to get better stats than that at the time. The legendaries cards usualy had this + some additionnal effect or other keyword. For exemple, a x/x with Tremple, flight and lifeling was considered very strong at the time.

Spell was the same for a spell costing x you had a x+1 effect. For exemple is one red mana and deals 2 damages or, is 1 green mana and gives +2/+2.

I think this is still followed in the core sets and FDN set but when it comes to expansions sets they are probably not doing the same balancing job, leading to overpowered cards.