Posted 26 November 2012 - 12:40 PM
You cannot 'downgrade' DX11. If you have Windows 7, you have both DX11 and DX9c. It will run in whatever DX version the software is compatible with, in this case, 9c.
When they implement DX11 functionality, you will likely have an option of choosing which to use.
DX11 will(ostensibly) offload more processing power to the GPU, meaning less CPU load(which is most people's bottleneck, currently). If you have a crappy video card but an acceptable processor, it will likely be in your best interest to run it in 9c.
With respect to .Net, you're fine with 4. If you have Windows 7, then 3.5 is installed by default and cannot really be removed. 4 is an optional install addon, which is why it appears as uninstallable in 'Programs and Features' and 3.5 does not.