Modern is often referred to as a “Turn 4” format. This means that aggro and combo decks aim to win by this turn whilst, on the other side of the spectrum, control and midrange decks attempt to stabilise before this point. Overall, the format feels significantly faster than Standard, but about a turn to a turn and a half slower than Legacy.
As a result of the “Turn 4” philosophy the mana curve of the most common decks is very low and streamlined. Many decks are saturated with cheap spells that cost one or two mana. These could be removal spells such as Lightning Bolt, Path to Exile and Abrupt Decay, or these could be aggressive creatures such as Goblin Guide, Delver of Secrets , and Tarmogoyf. It is important to have cards to play at all stages of the game, including turn one, whether that be in the form of draw spells, removal, discard, creatures, or counterspells. Sometimes the failure to play any cards even as early as turn two results in an un-winnable position where no chance of recovery is possible. This occurs when the player finds they are consistently one step behind the opponent and unable to react to all the cards they have played against them. Perhaps the opponent has too many creatures for the player to deal with before they die, or the opponent has a hand full of counterspells whilst the other player lacks any threats in theirs.
In order for spells that cost four mana or more to be considered competitive in Modern, they need to either be game-winning or provide a significant impact regardless of the game state. Examples of such high-cost spells are Scapeshift (a game-ending sorcery that results in lethal damage from lands entering the battlefield), Cryptic Command (an extremely versatile spell that can stymie a large attack or serve as a hard counter), and Siege Rhino (a creature that provides lifegain to stabilise against aggressive decks and trample to blast through weak blockers for a quick finish). All of these cards are popular because they result in an overwhelming advantage regardless of whether the player is ahead in the game or behind. Despite the power level of cards that cost large amounts of mana, many decks prefer to include a great number of cheaper plays in order to maintain parity with their opponent. This is simply due to the “Turn 4” philosophy and the high tempo of Modern; many decks can win on turn four, and therefore including spells that cost four mana or more is a risky strategy because it is possible for the opponent to win before they can be cast!
Some decks choose to avoid this pseudo-restriction in various ways. Spells that include phyrexian mana such as Dismember cost less to cast than it initially seems because players often cast them using life instead of mana. Mana acceleration is another way to be able to play very resource-heavy cards early on in the game. Decks such as Scapeshift and Tron try to accelerate their mana production with cards such as Birds of Paradise or Urza’s Tower and the other Urza lands (Urza’s Mine, and Urza’s Power Plant) in order to resolve powerful spells and develop their board state more quickly than their opponent. A third way is reanimator strategies with cards such as Goryo’s Vengeance and Unburial Rites that attempt to cheat incredibly large creatures such as Griselbrand into play without actually casting them.