1. Just the right balance between vulnerability and murderous aggression.
Examples: Timebomb, Baseball Bill (Avalanche version track 8) ,Razors Edge, All I Want
2. Call and reply style counterpoint, between all four in the original band.
Examples: I'll Fly Tonite, No Dark Things, Angels & Devils, In Bluer Skies.
Most alternative bands I have seen fall down on one or the other, and even the Bunnymen lose their powers when they depart from this formula.