In my mind, Disney Animation's problem was always that they had no idea how to do 3d. They either ended up too stylized or too blocky with their models and animation, and it appeared that the story suffered either because of lack of time or that parts were avoided due to these issues. Once Pixar came into the fold that all changed, they now had one of the top CGI companies in the world working with them. They were taught new techniques and most importantly I assume given assets that Pixar had.
Planes was not a Pixar movie it was a Disney Animations movie, even though the assets are a clear evolution (I'm being generous here) from what was represented in Cars. The models are strikingly similar; lighting, rendering (RenderMan), and most importantly the shaders. This carries over to many of the other hits that have come out of Disney with Tangled and Frozen having very similar model styles to each other. Disney has always been able to create a story, now that the technical hurdles are being addressed by Pixar (as they have always excelled at) they can focus on what they do best.