Regarding the WC talk, it's been said here before. WC is when you are between the top players of the world at your position. U may not even be the best at it, but there's no one much better than you right now.
So at Juve we have:
Buffon - WC and legend (best gk in Serie A);
Chiellini - WC (best defender in Serie A, he's back to his form that made him one of the best in Europe);
Barzagli - very good defender (but no WC IMO, needs to excel in Europe. But is the 2nd best CB in Italy, just behind Bonucci)
Litch (RB)- WC (best right back in Serie A, one of the top in Europe);
Isla (WRB)- WC (he's not completely proven, but very few players in the world can perform the way he does as a WRB. He's a very good player who plays in a position where very few guys can excel. So he is WC in this RWB position. Obviously the best in Serie A)
Asamoah (WLB) - very very good player, just need some time to establish himself as a WC player in this position. Top in Serie A.
Pirlo - WC and Legend
Vidal - WC - right now he's one of the top DM in the world. Actually call him DM is unfair. He's an all around midfielder, top in the world.
Marchisio - very good player, but still needs to show his stuff in CL to call him WC. Definitely top in Italy.
also:
Bonucci - he's developed a lot, and is important for the scheme and also in national team. Doesn't have WC potential IMO, but is top in Italy.
Giovinco - good player, can become very very good. Top in Italy. But I don't think there's WC potential.
Vucinic - good player. Can be very good.
For the future, we have also Pogba and Marrone, who have WC potential, both of them.