In an ideal world I'd hope for Nexus 2 to support every major operating system, game console and mobile computing device out there, but I doubt very much it'll happen. It's a (relatively) small developer with a (relatively) small budget and making your game work flawlessly on multiple platforms (such as Windows, Linux and Mac) is always a headache. I'd prefer them concentrating on gameplay, bugfixing, great mod support and tools and just general polish instead of spending months on porting their DirectX engine to other platforms. Going multiplatform makes the most sense, if you plan for it right from the start... especially if it's only a "small" indie game project. Nexus, however, is a rather complex title and the engine was built for Windows/DirectX, so there'd be major work investment involved to port this over.
... however, I'm a Windows user at heart, so I'm probably somewhat biased
Regarding DX10/DX11:First of all, Tesselation doesn't have anything to do with Anti-Aliasing: It's a technique to generate actual geometry (= more triangles/polygons) directly on the GPU. This is very useful for dynamically adjusting the level of detail of objects based on their distance to the camera, for example... or for generating really fine-grained details on the fly. Popular examples include the bricks of a wall made up of actual polygons and not just being a flat texture on a large, flat surface. Have a look at this video and notice all the detail on the stones and surfaces:
Multi-threaded rendering might prove very useful indeed - with DX10 and below you have to do all your rendering on one CPU core. Well, the rendering mainly happens on the GPU, of course, but someone has to talk to the GPU telling it what to render next (as well as to transfer data to the GPU and/or VRAM such as textures, triangles, shaders, etc.). Previously you could only talk to the GPU from a single CPU core... with DX11 you can split the work to multiple CPU cores, letting your engine make better use of multicore CPUs.
There are a couple of other interesting features in DX10/DX11, such as Compute Shaders doing all sorts of expensive calculations on the much much faster GPU instead of the CPU and other, more subtle features like anti-aliasing support for multiple rendertargets, etc.
The downside is, of course, having to stick to Windows Vista and above when making your game exclusively for DX10 or even DX11 while lots of people still use Windows XP. You're effectively going to lose customers this way, if you don't also plan for a DX9 renderer, further adding to the development time and costs.
