Absolutely ridiculously good conditioning - no bodybuilders today can match that combination of mass and conditioning. Dorian was probably the best Mr Olympia ever.
Anyone who slates his structure doesn't know shit - "structure" is almost irrelevant when you've got gobs of lean hard muscle in all the right places and it shows how good he really was - with a torn bicep and a slightly wide waist he still blew everyone away. Besides, his structure was pretty good - his legs didn't overpower his torso or vice versa, nor did his waist take any attention away from his shoulder width. From the rear, his back thickness didn't take anything away from how good his glutes, hams, and calves were.
Shame about the injuries, otherwise he would have had 9 or 10 Olympias easily.