A couple things:
He makes contact.
He doesn't play the ball, meaning his elbow has no reason to be there unless he's purposely trying hurt Reid.
That is violent conduct.
Violent conduct has a three match ban.
Seems pretty straightforward to me. It may seem harsh, but that's only because players get away with stuff like this behind the ref's back all the time. The pushing and shoving in the box during corners has always been an example of this, and one that the EPL has made clear they're trying to snuff out. Like it or not (and I like it) this is the norm now.
Sergio Aguero has been banned for 3 match following his conduct in the game against West Ham. Now, the question of today's thread is, did Aguero really meant to hurt his opponent, did he even made contact with his opponent and above all else, did he really deserve the three game ban?
Article on Skysport: http://www.skysports.com/football/news/11679/10562867/sergio-aguero-to-miss-manchester-derby-after-being-hit-with-three-match-ban
And video of him elbowing Winston Reid (sorry for the poor quality):
So, is it the right call or the wrong one?
May the debate begin!