Firstly, I don't use Mode S and not been taught a great deal, so this may be fundamentally flawed. No idea what the 3 characteristics are, but what they might be:
Mode S (unlike Modes A and C) isn't dependent on a pilot setting a squawk on his transponder. Each aircraft has its own personal identification so that means less risk of people setting the wrong squawks, misidentification, etc.
From an Area/Approach point of view, it provides much more information which helps the controller work out what's going on and also help improve safety. eg. If you tell an aircraft to descend to flight level 100, Mode S allows you to see both the level the aircraft is currently at, and the level that is set. If the level set is FL90, you can go straight back to them and ask them to check their level. Can help avoid level busts, etc.
Good for stacks - provides a 'vertical stack' image - otherwise on radar a stack looks a bit messy as everyone's flying over each other.
I'm sure there's plenty on the internet on Mode S, where it's used, etc etc - have a look!