Passion is the most overused word in football, I hear it all the time from English fans in respect to players and managers. Different people react in different ways. Does Buffon walking off smiling when he greets opposition players make him less of a player? What about Del Piero when he just walks off after a loss, he doesn't show any passion?
What I saw a lot of in our players after games this season was resignation, that was all of them not just ones we want to pick on. When the loser mentality is lifted we'll see better from all, but you take the rough with the smooth, can't just focus on extremes.
To answer your other question he is most dangerous making late runs into the box, but he's rarely there because he is off playing a restrictive left midfield role, or playing the water carrier role in the centre. The answer I don't have is where the guy can actually play in a modern system to make the most of his abilities. In a 5 man midfield as the offensive LCM or point in the 4-3-3? He doesn't have a shot like Scholes or Gerrard or the passing to play deeper but does link well with forwards. Not that this is the real challenge, the challenge is to create a strong Juventus before a strong Marchisio, the rest normally follows, but he either fits in or he doesn't.