When Magic the Gathering players talk about broken cards, they mean ones powerful enough to dominate games so that every competitive deck either includes the card or focuses on countering it. These cards break the game by destroying the options that make playing fun. D&D includes some spells that suck fun from the game, but nothing that ruins it.
Still, foresight breaks D&D. I don’t mean that it’s over-overpowered; 9th level spells should deliver wahoo powers that feel a bit overpowered. In the words of lead designer Jeremy Crawford, “High-level spells are often just whack-a-doo on purpose.” (I’ll bet he didn’t expect to be quoted on that one.)
By broken, I mean that foresight makes D&D dull, and a 9th-level spell should always add excitement. Even if the spell’s power wrecks an encounter, the magic should feel game breaking and thrilling. Foresight just feels game breaking and boring.
For 8 hours, the target of foresight “can’t be surprised and has advantage on attack rolls, ability checks, and saving throws. Additionally, other creatures have disadvantage on attack rolls against the target for the duration.”
Fifth edition’s rules for advantage and disadvantage streamline the game by eliminating the fiddly pluses and minuses that older editions imposed on attacks and checks. While the old modifiers added realism, they slowed play and seldom made a difference.
To gain even more quick simplicity, multiple sources of advantage and disadvantage don’t stack. At most they offset, leading to a straight roll. Usually that works because multiple advantages and disadvantages come infrequently. (For instances where this rule creates illogical situations, see numbers 12 and 10 of the 13 of the Craziest Quirks in the Dungeons & Dragons Rules.)
The simple approach falters when some ongoing factor adds advantage or disadvantage to every roll. Suddenly other circumstances stop affecting the odds. D&D’s designers recognized this when they opted to have cover impose a minus to attacks rather than disadvantage. They wanted a factor as common as cover to stack with all the other situations that can create advantage and disadvantage in a fight.
During my D&D weekend, when two level-20 wizards benefited from the 8-hour duration of foresight and spent an entire adventure with advantage, I realized how much less interesting the game became. Foresight eliminates all motivations to seek an edge. By erasing fifth-edition’s foundation of advantage/disadvantage, foresight nullifies the effect of too many decisions, tactics and character traits.
Incidentally, the 3rd-edition version of foresight gave just a +2 bonus to AC and Reflex saves—at 9th level! Instead of a broken spell that sucked the fun from D&D, the spell just sucked.