Shadow is spot on.
You can't argue 'if Mandzukic was faster...' because he wasn't. If he was it would be a different situation warranting a different call. It doesn't change this situation.
Ramos slid alongside Mandzukic. Not at him. The slide wasn't dangerous, it was Mandzukic's kick, which was late, that was dangerous (mostly to himself, but also to Ramos).
Again, it is all about who initiates the contact. In this instance it is Mandzukic who does so. It is clear to see. He tries to kick the ball, but is too late and kicks Ramos instead. If he doesn't kick, or pulls out, there is no contact.
I would have understood had the ref given a penalty, because it was a very close call. The first couple times I saw it I thought Mandzukic got the ball first and Ramis played the man, but in slow motion it is clearly the other way round.
In response to Lukas. Dangerous play covers anything where contact is not made. If this was called a foul--say if Mandzukic did get there first--then it would have been adjudged as 'kicking' by Ramos. Resulting in a penalty kick.
Rule of thumb is, if there is contact, it's not dangerous play, and not an indirect kick.