FIFA Law 12: “Reckless” means that the player has acted with complete disregard of the danger to, or consequences for, his opponent. A player who plays in a reckless manner shall be cautioned.
This seems to sum up fairly well the two incidents above. Both Diego López and Nani were trying to win the ball but both of them went in without considering the consequences for their opponent. Nani caught Alvaro Arbeloa with his studs, Diego López punched Nemanja Vidic in the head with both fists.
Nani got sent off and López wasn’t even booked.
FIFA Law 12: “Using excessive force” means that the player has far exceeded the necessary use of force and is in danger of injuring his opponent. A player who uses excessive force shall be sent off.
Whilst both Nani and López were in danger of injuring an opponent, neither of them used excessive force. This also rules out the sending off for “serious foul play”: A player is guilty of serious foul play if he uses excessive force or brutality against an opponent when challenging for the ball.
If we follow the Laws of the game, rather just opinion based on nothing other that what people personally believe should warrant a red card or not, at worst, Nani and López should have been booked.
FIFA Law 12: If a player plays in a dangerous manner in a “normal” challenge, the referee should not take any disciplinary action. If the action is made with obvious risk of injury the referee should caution the player. If a player denies an obvious goal-scoring opportunity in a dangerous manner the referee should send off the player.
Graham Poll: Nani challenged for the ball with a raised foot and only had eyes for the ball but caught Alvaro Arbeloa. It looked like dangerous play and at worst a yellow card.