I think their defense is better, and their offense didn't lose as much as people think. Players have stepped up, i.e. losing Pittman isn't a big deal when you have Chris Wells stepping in. Brian Robiskie has stepped up as receiver, and now Ray Small is healthy and playing like a Ted Ginn Jr. replacement. But mostly it comes back to the defense.
We'll see come bowl season though, I'm not sure we're going to learn much more about Ohio State in the regular season, although I could be wrong. The game at Penn State could be tough, and Michigan is obviously never an automatic win. And if they get Wisconsin's best shot they will be tough.