I think the one thing that helped Stevens when he was hired was no established stars. You can count Rondo, but he was injured, and then came back, and then shipped out. Stevens has led this group of misfits that really have had no say in the NBA, and really had no leverage on a team. Now, there's no doubt about Steven's coaching ability. I think he's amazing and a top 5 coach in the league, but when you come into that situation you can really assert your leadership and your style.
If Stevens goes to OKC, will he get the same results out of an MVP candidate year in and year out? Will Westbrook be as willing to buy into the team first, selflessness style of play Stevens promotes? The celtics had to 3 years ago. Westbrook doesn't necessarily have to.