While it may be old news to some, this is the first time I've seen that Constitution is a one time hit point modifier. Under the 3.x rules, 18 Con vs 8 Con at 30th level would be a 150 hit point difference. If it is now merely a 10 hit point difference, this is (to borrow an MMO term) a massive nerf to CON (and in a roundabout way, a massive buff to the "Toughness" feat that we've seen). Additionally, if your Fort Defense is based on the highest of either your STR or CON modifier, I don't see any particular incentive for Defenders to worry about massive CON scores. This assumption precludes requirements for Feats, Paragon Paths, Epic Destinies, ect.