Cards that let you play a spell when you discard, like Loxodon Smiter, were designed as an answer to discard effects like Liliana of the Veil
Sword of Fire and Ice Card draw and a free Shock with every hit landed on your opponent
Its especially useful with effects that care about land types like domain or converge and has found a home in decks looking to combo off with Valakut, the Molten Pinnacle
Opalescence (Ext, S) The weaker half of the Replenish-Opalescence combo, nevertheless this card terrorized the Extended and Standard scene for long enough
In deck building, this balance is called the "mana curve." Each spell has a mana value, which is the total mana cost of a spell
Many dragon-based decks are already able to amass a punishing engine with the likes of Sarkhans Unsealing, Dragon Tempest, and Scourge of Valkas