Mode S is hardly new, its been around for a quarter of a century now! I'm always amazed at the reluctance to accept new technology once it has be proven to be reliable and of benefit. I can only imagine its that old chestnut of cost making people reluctant to use it.
As stated above mode S is basically A+C with automatic aircraft ID as far as ATC are concerned. In the air its function extends to the aircraft collision avoidance system, a very reassuring bit of kit to have I must say, having had it in my professional life. The more aircraft 'S' equipped there are, the better things are, especially when venturing into "no mans land" outside controlled airspace! ADS-B is the system where there is downlinking of more data for tracking purposes (present position, track, ROC/D etc. etc.) Not sure why that's a privacy issue for private flying though.