Is there a difference between the CyanogenMod 11S Rom (for OnePlus One) and regular CyanogenMod?
AndroidPit has an article that outlines the major differences.

http://www.androidpit.com/cyanogenmod-11-vs-cyanogenmod-11s-the-main-differences


Google Apps, no root
Extra Themes and Wallpapers
Display activation via tapping
Optional on-screen buttons
Screen calibration and adaptive lighting
Camera and Gallery App
Boot Animation
AudioFX instead of DPS Manager


Other than this, CyanogenMod 11S is natively tested on the OnePlus One and contains the best possible drivers for the phone. Essentially, it is optimised to run on the OnePlus One..