After seeing the amazing Pandaren models and animation I for one am glad they are not releasing the new models for old characters at release. Why you may ask? Well I believe if they wanted to just update the models they could probably do it pretty fast, however if they update all the models and textures to mop quality and create newer animations, well that is more work.
So the question is this. Would you prefer newer updated models at launch or MOP quality Models, textures and animation but wait a little longer...