Barbarian is the only class that does something special with it. This is the part which I think is a lost opportunity. It would be nice if 1 or 2 other classes would have a reason to push to an 18 or 20 Con.
Yeah, Barbarians are the guys who will try to get CON 20, for the unarmoured defence, for the CON 24 at 20th, and for the doubled up hp bonus raging.