Well, if you don't mind moving away from C# to C++ you could check out HGE. It's a little on the old side but is very easy to use and extend. If you're willing to search the forums a bit you will find ports of it to DirectX 9 and maybe OpenGL (it has been a while). By default it doesn't support things like shaders. It does include sound and input, but no physics or collision detection. For that there is always Bullet or Box2D. For a turn based game or simple real time game that may not even be necessary.
As far as GUI goes its built in GUI is garbage, but you can integrate CEGUI into it. CEGUI is actually pretty easy to extend to different rendering systems, even if skinning it can be cumbersome. The implementation for HGE is two files and they are pretty short.
Clanlib may also be something you want to look into. I haven't used it personally but it appears to be quite powerful and includes a pretty straightforward GUI component. It is also actively developed while HGE hasn't been updated for a couple of years.