Durant's size makes him a better rebounder and defender than Barnes, so while not the traditional bruiser you might associate with as a fix to their problems, he's definitely an upgrade on some of their weaknesses. One of the reasons OKC pushed Golden State to the brink was the benefit of having Durant as 7 foot tall wing to clog passing lanes, bother shots, and help with punishing them on the glass. Now they're adding those benefits into the role that Harrison Barnes was playing in their "Death Line Up" and that does make them better.
Remember all those open jumpers Barnes was clanking off the rim in the Cleveland series? Now imagine those shots going to Durant. And even if they're not the same quality or quantity of open shots, the stress Durant places on a defense compared to Barnes will open things up for the rest of the team. It's frightening what their offense could be like.
I think the only fly in the ointment to the possible offensive nightmare machine that Golden State could be is how does Durant adjust to playing in their system. One of the few offensive weakness for Durant has been his off the ball movement. Some of it could be laid at the feet of OKC's simplistic offense, but even with all his freakish athleticism and skill for a 7 footer, he's never looked natural moving off the ball. So often players, or at least top guys like Kawhi, Tony Allen, are able to stay on his hip through screens, and then push Durant off his spot before even getting the ball. It's probably something that won't show much in the regular season, as Golden State's barrage of three pointers and passing isn't something teams can figure out in an one off setting, but at least could add some drama to the playoffs.