I think we have enough to appeal Durant given our cap space and plethora of picks to make a deal for another superstar.
Even without making any moves, Durant automatically makes this team a 55-60 win team who quite possibly gets by Cleveland and gets to the Finals. That's already more than any team out West can promise.
Then we factor in that the C's could add another two stars around Durant through free agency and via the draft or trading of picks, and you have quite a package. And quite frankly, while it may take another year, it shouldn't be hard to pitch another star or two to join a 55-60 win team with Stevens and Durant leading the way.
I also think the Boston situation is appealing to Durant because this will definitely be "his team," built around him. He's not dumb and can see how some view LeBron who couldn't get it done in Cleveland, went to Wade's team (which had already won a title), and now is back in Cleveland where it looks like he won't get it done anytime soon. So I think Durant will be very wary about joining an established contender where it will look like he's just riding the coat-tails of other stars to his first ring.
(And as a note, I don't necessarily think that the above assessment is true of LeBron, but there's certainly a contingent of people under that impression.)