What are the Mavericks missing?

In this regular season series against the Oklahoma City Thunder beat the Dallas Mavericks for the 3rd time in a row. The first two games the Mavs held them all the way to overtime but then failing. And the game last night was just a complete blow out. So my question is, WHAT ARE THE MAVERICKS MISSING? I realize they have the best scorer in the league (Kevin Durant), and they have all star point guard Russell Westbrook. But that doesn't mean that Darren Collision cant guard Russell, or that Shawn Marion cant guard Durant. We are a so much better team than we were last year, adding so many great key players. I just don't understand. If the Cavaliers can do it than so can the Mavericks.