A huge boost in the available intervention: You can set ISO, exposure compensation and/or flash compensation, and when to use or not use flash. Plus, you can shift the program to get different combinations of shutter and aperture.
Pluses: This is a good walking-around mode to use when you want to be ready for a grab shot but still have control for more deliberate shooting.
Pitfalls: Sets a floor for shutter speed, usually around 1/60 sec, though you can often override this in custom functions or other menu settings. If you shift the program to get a different shutter speed or aperture, it will revert to the camera setting after one shot.