Andy I agree you most of player evaluation but not this one.
Barzagli IMO always seemed to have that calmness about him which you can rarely find in young defenders. He had 3 decent years in Serie A already in his CV as well as decent contribution to Italia's WC winning campaign. For any defender that is an impressive resume but for 24 year old, I have to say he has potentially to be very best.
Beside, defenders in general can take up hell of lot more milage than attackers, and they tend to mature at later stage. I do not know the exact reasons, but my presumption is that the element understanding the flow of game matters more for defenders than attackers. Terry, for example, who is widely regarded by many to be the best defender ATM, was virtually unknown playing for England U21. Yes he was a promising defender if I recall, but anyone predicted his current achievement would have been laughed at 4 years ago.
I just think defenders' growth rate are unpredicted in most cases because they do not get the same limelight as the attacking phenoms.
Just looking at Barzagli's resume, it is as good as you can get for a 24 year old.
I agree with you that he is a risky buy but then again the question is at what price? Barzagli is Italian so there is no adjustment issue that comes with foreign players. And 10m, as reported by some press, IMO, is a bargain buy when dumb "me-cool" defenders like Rio gets paid over 100K pounds per week and gets traded over 30m.
The question is would he come? My gut feeling suggests he will probably end up in Milan sometime soon.