CharlieOS: Hello everyone. First, I just want to give a big "Thank you!" to everyone whose provided help to me in this thread. I very much appreciate all of your comments and insights. Now, after reading over all the comments, clearly my installation of System Shock Enhanced Edition was missing some key files and functionality (for example, my installation -- and re-installation -- never installed a controls.cfg file in the game's root). Mind you, I downloaded and installed my copy of System Shock Enhanced Edition through GOG Galaxy. So... I then tried an experiment: I downloaded the game again but this time, I didn't use GOG Galaxy. I then *manuallly* installed the game and, guess what? The manual installation DID install the controls.cfg file and it also allowed for toggling between keyboard and mouse, WASD movement, etc.!
So, for some reason, when I installed the game through GOG Galaxy, WASD movement, the controls.cfg file, etc. simply didn't install. Hmm. Might someone else here be able to install the game via GOG Galaxy to see if he/she gets the same result that I did? I'd be very interested if you find that the GOG Galaxy-installed version lacks WASD movement options where as a manually installed version of the game contains that movement.
And, again, everyone, thanks for your help!
I installed my version through Galaxy right away when the game was released and didn't have any problems. Is it possible you clicked on the 'classic edition' by mistake?