One key difference versus Shader Model 5.0 is that whereas this old model relies on the fxc compiler to both compile and optimize the code, the new model adopts a Clang HLSL compiler and direct optimization to multiple LLVM passes.
Since Shader Model 6.0 supports HLSL code from v5.0 upwards, it should also benefit existing DirectX 11 and DirectX 12 apps, while relegating older apps to the then legacy Shader Model 5.0 pathway. In addition, Shader Model 6.0 claims to provide the right performance to cope with API level features such as tiled resources (mega-textures). It remains to be seen how Microsoft deploys Shader Model 6.0.
Source: TPU