Ad Nauseam

The genius of the design is that it hands you the rope and dares you to stop pulling. There is no built-in cap, no enforced ceiling on how deep you go: the only governor is your own life total, and the cost of each card is its mana value, which means a deck of zero- and one-mana spells pays almost nothing to dig through itself. That single constraint is the whole balancing act. Build fairly and it is a slightly painful card-advantage burst; build to abuse it and it becomes the engine of a glass-cannon combo, where the plan is to draw your entire library at instant speed on an empty board and assemble a kill before the life loss matters. The interplay with effects that ignore your life total, or convert that emptied library into immediate damage, is where the card stops being a draw spell and becomes a win condition. Instant speed is what lets the whole thing happen as a surprise: cast it on the opponent's end step to untap with full mana, or fire it in response to interaction and execute the entire chain on the stack before anyone gets a chance to break it up. That tension (an effect with no upper bound, throttled only by how low you are willing to set your own life) has made it the centerpiece of dedicated combo shells named directly after it, and a recurring object of B&R scrutiny wherever those shells got fast enough to matter.