“Broken” gets thrown around a lot to describe Magic cards and strategies, but how can we define it? What common elements ruin games over and over?
Converting one resource to another indefinitely with no transaction cost.
New players are often reluctant to willingly pay life, sacrifice permanents, or discard cards. Sooner or later, the realization hits: They're all just resources to be used in whatever way is most effective.
Yawgmoth's Bargain and Channel are prime examples. Gaining and preserving life seems important, but it doesn't win games, it just postpones losing. Sometimes that's enough, when you're stalling for time until you can assemble a combo to win on the spot.
But there's a lot you can do with drawing 19 cards or adding 19 mana at once.
Phyrexian Arena and Greed are good comparisons to Yawgmoth's Bargain. Phyrexian Arena is gated by time: only once per turn, and requires a turn cycle to do anything. Greed is gated by a mana cost for the draw, requiring another less-available input. Both are still good cards, but they don't shatter formats.