One of the low-level changes is change in kernel. If you have noticed carefully, the Kernel of Unix/Linux is and was always in executable form. But this is not the case in Windows. Pointed by on of my friends, Mr Rama Krishna, Windows 3.1 had an executable Kernel. Since it was closed source, many tried to know the internal details of it. It was very easy to hack it. Open any debugger or hex-editor...and it is laid out.
Although Microsoft was beat there, it came back in form of Windows95. There was no formal way of getting it work, other than running the system. One more similarity is usage of DLL files. These are nothing but Shared Object .so files found in /lib. Exciting isn't it? If u find any more such "borrowings"....you know what to do.