The Thunder seemed to break the game open on a succession of Spurs defensive miscues. They repeatedly isolated the Spurs bigs in pick-n-roll situations and either beat them off the dribble or kicked out to open shooters. It seems that when the Thunder missed on dribble drive penetrations to the hoop, there was usually a Thunder teammate there to clean up the mess with a timely putback.
Meanwhile, the Spurs are crippled on the frontline because they're really playing with 3 bigs (Duncan, Splitter and Blair - Bonner doesn't count). The Spurs were getting minimal points in the paint and, at least of this game, couldn't keep the young Thunder out of the lane.
The lack of an interior paint presence, on both ends - but especially on the defensive end - was really alarming. Of course, it's a wart that has been exposed in the past two playoffs. It's only getting progressively worse.