I guess its not the models which have to be more high-resolution but there's some kind of displacement map for the textures. For complex models like the dragon this displacement map is created from a high-res model, of course. But for textures like the stones or the walls I think they tile it, otherwise it would be quite inefficient to create content.
Of course things will be tileable for repeatable patterns but just unlike for normal maps you cannot fake a good final look by just running some filters over the texture itself. You have to create it from a high-res mesh as it usually also gets done for normal maps (I don't think handpainting the needed maps is an idea that works). So as final thought you'd need a high-poly mesh for every single texture you paint...
What I don't get, though, is why this should be DirectX 11 only - aren't geometry shaders already present in DirectX 10?
They used to be in DirectX 10 but NVIDIA refused to implement them back then and so Microsoft agreed in delaying them. That also lead to ATI having next to everything since the HD2900 GPUs which might have been one of the reasons why those cards turned out so poor as lots of time was invested into integrating a "useless" element on the chip...
That also might have added as to why DirectX 10 did get so little attention as it in the end only delivered little improvements or new features. It's now DirectX 11 which introduces all of this and gives GPUs the possibilities to on-the-fly create such geometries. I'm not sure when this will make it into OpenGL - considering the failures of version 3 it probably will take a while...
