The thing with Barzagli is, he always had the potential to become a very good and reliable CB, he was quite highly rated in his younger years, he won the U/21 Euros with Italy in 2004.
His career sort of stagnated by staying at Palermo for too long, and maybe the move to Germany was not right for him (even though he won the German league in his first year). Like a lot of Italian players, he is playing his best football at a relatively "old" age. Anyway, he's playing great these days and is very solid and commanding in defence for us. Maybe the trust Conte has in him has made him a better player?