If anyone on here actually bothered to read the laws of the game available from (Fifa.com) it's quite obvious that the challenge on aguero should have been a sending off for 'serious foul play' under law 12. The interpretation of this law is shown on page 128, which is page 130 of the pdf document:
It endangered the safety of the opponent and it was a one footed lunge from the front with excessive force and brutality
The first 3 criteria of the guidelines were met.
Yaya's challenge was deemed reckless by the referee, which is a bookable offence. He acted with disregard to the dangerous consequences but wasn't using excessive force i. e. wasn't in danger of injuring his opponent.
The referee also saw Dann's challenge on Aguero as the same (page number 119, 121 of the off)
Compare a reckless challenge with serious foul play.